Hey Charlie,
Thought I'd throw in some thoughts to confuse you more. I'm 102 kg (ISH) and have done some testing in this area comparing the 07 F2 SX xs (80 litre) and the 07 missile s (63 litre) a few years ago . Both on the speed couse and in the bay, they are very similar in aound 25 knots. Side by side testing I did constant 42's on both boards at Sandy Point on flat water. In the bay in 10-12 "chop, 38-39's in the same conditions. I usually run the 6.6 koncept or even the 5.8 if its a bit above 25. The Sx does plane earlier and the missile is more comfotable at speed. Worth noting that the missle s is definately faster in stronger winds, and seems to top out around 47 kn. If you have a nearby speed strip and you have the speed bug, get youself a missle xs. At 75 kg, it'll go in 25 kn and has no top end limit. It will still take the 6.3 but prefers 5-5.8. If you're looking at 20-25, the missile s will easily get going with a 6.3 and is surprisingly "slalomy" but in these coditions, a good small slalom board will be just as quick.
the short of it? If you have a great speed strip nearby, go the missle (either one, depending on ave windspeed) but if you are in the bay or ocean, go the slalom.
confused?? Get a big van and get em all

cheers Tony