I can add some rambling thoughts here about the two skis. I had the Epic V10 sport (g2) for 2+ years and have had a SR (g2) for about a year. Unfortunately I can't fully address the two's difference in flat water speed as I really only used them if rough water/downwinds and didn't really care much about flatwater speed in this class of ski for me. That said... just by looking at them side by side I would put my money on the V10 sport. It has a longer waterline, is flatter/ less rocker and has a less of a 'full' shaped hull than then SR.
But!... (there is always a but) .... in my limited experience on flat water with a GPS and trying to sort out which was faster it was somewhat inconclusive. My feeling is that the SR is a tiny bit faster... but there were a lot of variables in the 2-3 times I was doing this. In the end it doesn't really matter for my purposes so I didn't bother trying any more. Current where I paddle, fitness, and how I feel that day all can lead to vagueness on a real idea on which ski is faster in the flats when they are so close. In the bumps however, I'm clearly faster in the SR.
The estimated .5 mph speed increases over the V7 seems reasonable but I have never paddled one. I think Greg's estimation of speed difference is too little between the V7 and the Sport at least for me. I have average at best speed in flat water and find something like a .2-3 mph differences between ski 'classes'... i.e. the SR is about .3 mph faster for me than in the S18s (same class a V8) and the SEL about .3 faster than the SR... The V7 being quite heavy and definilty more burdensome than the S18s would almost surly be significantly slower than the S18s.. so .5 sound about right for me.... purely speculative numbers on the V7 though having never paddled one.
Stability wise I find the SR and Sport similar but different. The sport is more stable initially feeling solid and twichless. The SR is looser initially and sits a touch less 'flat' on the water. But .. (there is always a but) the SR has more and deeper secondary, it's progressive and predictable. To me the Sport has less secondary and is somewhat vague and not progressive. After 2+ years in the sport it always felt somwhat iffy in conditions where I was relying on deep secondary. Like surfing aggressively diagonaly in confused steep waves where you really need to keep the power on, or in bad and big reflecting chop. The SR on the other hand is confidence inspiring in those conditions leading to me to being able to put more power down, throw less check braces and focus on paddling aggressively and not my stability edge. I see them as being roughly similar in stability over all but it comes in a different places for each ski. For flat water ether is fine and the Epic might have a slight advantage being more stable in the first few degrees of tilt. In reality though they are both quite stable in the flats and it shouldn't really be a deciding factor.
Fit might be the biggest differences. The buckets are very different. Epics being more rounded and for me quit a bit more comfortable. I suspect without me knowing Epic took a mold off my ass, it's perfect.. the fit, not my ass. But everyone is different so you might fit the SR better.. who knows. The SR's bucket is larger and I needed to add a lot of padding here and there do make it fit me. Seating position for me goes to the SR by a touch. A slightly higher butt or lower heals feels better. Less of that paddling up hill feel.. it's not a big difference though. The stellar footplate assembily is much better than Epics.. it's really solid feeling and doesn't flex or sqeek at all. It also has finer adjustments which I really like. But!... (there is always a but) ... Stellar still has a big cockpit. It's smaller than (g1) boats but it still holds significantly more water than ... well, everyone else's. The new twin bailers are adequate..just, but only if you fill the empty space with foam. In flat water it's not an issue.
The deciding factor might be layup. Epic doesn't offer a "multi sport" layup as far as I know. Stellar and Think both offer a "multi sport" layup for there boats (so look at an Evo III as well, it has similar speed and is also a great ski) with an over the stern rudder mount standard and flush mount bailer options... that's really nice for flatwater. Epic has a flush mount bailer standard.
Really, fit and layup I think might be your two biggest differences between these two skis in practice on flatwater. In the bumps or rougher water things change significantly though. If you are really only paddling in flat water, you might also want to look into a V10 or Stellar SEI.... both are a good bit faster than the Sport and SR and in relation to flat water not that much harder to paddle... I'm talking actual flat water here.
As to the SR (g1) vs. whatever..... the SR (g2) is a better boat in every respect to the first gen. It's essentially a totally different ski in practice and there are no downsides other than a loss of stability, It's not profound but it's there. It surfers better, has a much better bucket, drains better and catches runners much better. It's a super ski for surfing anything but at its best in smaller to medium sized bumps in wind chop as it's short and very maneuverable. I like it much better than my past Sport as it is much more maneuverable on a wave and catches runners significantly easer. It's just more fun to paddle. Stellar moved the seat forward 3" from (g1) and it really made the ski. But!... (there is always a but) .... it really needs a better rudder than what stellar offers. I did not like the ski (downwind) with the stock 8" rudder, at all. I put on a 9" downwind DK rudder onnit and it transformed the ski, no hyperbole. Don't get me started on the state of rudders from all ski manufacturers.... there is not enough room on this page for that rant.
I hope that helps some..l good luck.
Bill
After re reading this I just want to be clear. I don't think there is a stinker ski out there right now. It's all about finding a ski that suits your body and paddling style.
For me Epic doesn't really work for my personal paddling style. The skis don't turn when and where I want and and has a stability profile I have never warmed up to (the new V12 looks sweet though!) . But... (there is always a but)..... My frustration with Epic doesn't stop all sorts of people kicking my ass on a regular basis with there Epics without complaints about stability issues or seeming to have any issues turning their skis. So again, I'm not bashing Epic at all. They have more than enough happy paddlers and for good reasons. It reminds me of white water kayaks... planing hull vs. displacement, speed vs loosens and volume distribution. Everyone likes something different and when everything comes together and you find the right mix it's amazing. But there is your buddy on the same river who has just the opposite mix in his kayak, swears buy it and is killing it. Moral of the story.... try lots of skis yourself and take what every says with a grain of salt. They are all good, it's just finding the right ski fo you. This basic advice would have saved me a lot of money over the years.
FENN Bluefin S
FENN Swordfish S carbon hybrid
Epic V8 double gen 2
Lot and lots of DK rudders.
Had:
Stellar SEL excel (gen 2)
Stellar SR excel (gen2)
Stellar S18s g1 (excel)
Epic V10 Double (performance)
Stellar SR (gen 1)
V10 sport (gen 2)
V10 (Gen 2)
Beater SEL (gen 1)