I have owned and paddled a V10L Ultra for close to 2 years and loved it. For the past 8 months I have paddled the V12 Ultra and now this is my ski of preference.
For me the flat water speed according to my trusty Garmin is pretty much as Red Pepper suspects, slightly faster with the venturi shut but about the same open. It is only about 0.1 to 0.2 kph faster on flat water distances above 10ks (at an my race average of just over 12kph), but in a sprint the V12 appears to have up to 0.8kph more top speed.
As with AR Convert I have also found the primary stability to be less than the V10L, but the greater secondary has saved my bacon many a time in rough ocean conditions when the 10L would have had me swimming.
Learning to trust the V12's greater secondary when the primary makes it feel livlier took some time.
It's no trouble in the flat but out in the ocean swell and chop it took me a good 2 months to get comfortable with it. Which is worth noting for any one that makes a decision after a 5min paddle. Stick with it and you will be rewarded.
Initially I tried the V12 when they first came out for about 2 weeks and happily went back to the V10L as I did not believe I could handle it in the rough (with the livlier primary) as well as my V10L & I had trouble working out the different way they handle.
I have now pretty much worked out the technique differences of the 2 when chasing runs.
The wider tail of the V10L means that you wait for that feeling of lift from behind and then power on late using the lift to accelerate. This still works well enough when you are tired.
On the V12 you look for the hole forming infront of you and go early using its extra top speed to power into the hole. The V12 then rewards you by carrying slightly more speed on the run and for longer (helping to link the next one) getting ahead of the V10L but can wear you out if your timming is off and does not work so well when you are tired or not fit.
If you paddle the 12 like the 10L and try to go late when you feel the lift, the narrower tail and more nose volume does not give you as much lift from behind you may miss the run.
2 distinctly different techniques but I guess you get used to what you paddle if you paddle often.
Just to check my prefference again, I paddled the V10L the other day and whilst it is still a great ski they just do things differently and I could not quite nail my technique and timming as I now can in the 12 in the rough or flat.
For the record I am 175cm tall and 78kg. My V10L Ultra weighs in at 10.5kg and my V12 Ultra is a little heavier at 11.5kg.