For flat water you can get the smallest weedless rudder. I have it and the large elliptical surf rudder. I change b/w them occasionally. I only have the new V10 now (sold the Sport before I got the new 10). I don't measure precisely enough to see if the smaller rudder makes a difference in speed, but I usually keep the big rudder on - it has more bite for catching boat wakes, which interests me more than top speed on flat water
For the very occasional race I do on flat water, I may put the small rudder on as I'm sure it has a bit less resistance.
The V10 tracks good, almost too good IMO, so with the big rudder it is even less lively (though it is obviously more responsive to rudder input with it than with the smaller ones). I think it just feels dull to paddle (even though it is faster), compared to shorter and more maneuverable craft like your V10 Sport. I also have the Think Eze, and that is sooo much more lively and maneuverable! That's not necessarily always a good thing as it zigzags a lot for no good reason, but comes handy for maneuvering on small wind waves.