my take and general experience is the following:
If I paddle an elite ski (Elite S) at 12.5 kph on flatish water, then my speeds at similar efforts would be
- a intermediate ski (Evo, Vault etc) the flat water speed is 0.5-0.7 kph lower (typically 11.8-12.0).
- a "beginner/stable" ski (eg Fenn Xt) the speed loss is about another 0.5 kph (11.2-11.5)
That is a time loss of about 2-3 minutes per hour.
(I know, not statistically valid or adjusted for wind, bump size, current and precise HR ...)
Down wind speed is all dependent on you ability to maintain an equally efficient stroke, particularly at the difficult spots ( at the top of the wave, ). If you miss that stroke, you miss the wave and your speed drops dramatically. eg. 10 instead of 15. So DW stability you may want to use a stable boat to catch/link more waves and get a higher speed and more fun.
So what is the faster boat? That depends on the paddle condition and you ability to put in 100% paddle efficiency under those conditions. I agree that we tend to pick a too tippy boat given the actual conditions of the day. Hear Hank McGregor´s comments about the new V12 vs the V14.
I believe theoretical drag calculations show too small differences between the different stability "classes". Possibly because wider boats gives you a slightly less efficient stroke (catch distance from center, seat height, wave slapping ....... I don´t know).
Again, my experience is that this comparison will not hold true if you try more unstable boats than you normally paddle. For me I found that trading to more unstable boats gave me no (maybe even a drop in speed) increase in speed until you body adapts to the new boat. If ever - I have never managed to paddle a 39 cm K1 faster over a 5-10 distance than my elite surfski.