I know this is an old post but thought I'd add to it incase anybody else is thinking about buying a Viper.
I have just purchased one after trying pretty much everything else on the market and weeks of deciding weather it is the right boat for me. I'm a fairly inexperienced paddler and was initially looking at something more stable but I do most of my paddling on flat water and still have my old ski for rough days. So far its been great and I feel I made the right choice.
As a starting point for a review, read the review on the Robberg Express. Comes from the same company and has similar lines under the water although this one is longer and the seat is further back.
Stability is similar to the V10, more stable than the Robberg Express and Fenn elite (in my opinion). It has a slight flatspot under the seat to help with stability
Seat comfort is a major reason why i bought this ski and it is very good for me but it is a narrow bucket. The seat is raised to keep the bum above the heels. As an indication, I have a 33 inch waist and the boat is a good fit for me, much tighter and nicer fit than the Fenn's and Epics for me. I've had no problems with it rubbing on the lower back (coccyx) which I found a problem in some of the other boats which i assume is due to the back of the seat been slightly cut away.
I'm not the best person to ask on speed yet but I noticed a guy down in South Australia has one and has done quite well in a few races. I assume its pretty similar to anything else on the market but its the paddler not the boat more than anything.
Footplate set up is very good and easy to adjust similar to Red 7.
On the water, it feels to have a lower volume than other skis but off the water, they all look to have the same volume.
Turning is good, due to the rocker in the hull
Build quility feels pretty good from a reputable South African kayak brand and the price for a new glass ski here in Australia was the lowest of all comparable skis on the market.
Hope this helps. Above all else, test paddle as many ski's as possible. Everyone here in Sydney was extremely helpful and all had good opinions and advice.
Cheers