I'll have to find you at trike fest some time, I have both the 250sx and a 350x =). My 250sx is slow as can be acceleration wise vs my 350x, I'd put it more on par with a 200 series 3 wheeler. My 350x doesn't top out by the end of the track, but I was only a couple lengths behind a guy that had a 490 big bore kit in his, and I'm racing him with my crappy looking machine that probably is a stock engine setup (I never opened it) with a overly loud DG exhaust and a poorly tuned carb (lean). The SX did take a long distance to max out, shifting defo takes away from the acceleration, belt drive is more ideal for drag style racing because of it's CVT effect. Not sure how efficient it is for drive line losses vs say chain drive though. I suspect I could keep up ok on my SX vs your P3, but doubt I'd beat you, I'd have to power shift to even have a chance. I wonder what the HP is of the P3 engine. 2 stroke 250cc should be somewhere near the power of a 250r or 350x.
Ironically, when we were getting the top speed of my 250SX, my cousin was also doing the same for his 1987 TRX250X, his machine was a little faster than the SX, but wasn't a huge difference, his top speed was higher though, maybe like 55-58mph.
Anyway, I don't exactly ride my 250sx because it's the fastest machine, it's just really nice being shaft drive, mud & jumps etc isn't a problem, just can't do any extreme jumps since the suspension is too stiff and probably not nearly enough travel.