The max speed of the v10S will be higher but you need to paddle pretty fast before you'll notice much difference, because at slower speeds (like less than 6 mph) the friction drag from wetted surface area is more of a factor than the displacement drag created by pushing a hole through the water. Sorry can't provide any hard numbers to back that up, not much of a physics wonk.
Never tried a V8, but I paddled a v10S for a season and it was great. Plenty quick on the flat and I found it very stable, like eat a sandwich stable in flat water. I was actually faster on the sport than my previous beater fenn millenium because of the stability/stroke improvement.
When you say it's hard to test paddle boats prior to buying, do you mean the seller won't let you take it for a spin prior to laying out cash?
