The FX HO accellerates very quickly. There is no hesitation at all (tell your passengers to hang on). I've never measured it, but I'd say that you are at 40 mph in just a few seconds, then it takes longer to gradually reach top speed from that point.
The most popular upgrade options are from R & D Performance
R&D Performance Products USA and RIVA Motorsports
Welcome to RIVA Motorsports on the World Wide Web!, but there are others as well. I will probably buy from these guys:
WATERCRAFT, since they have good prices and post helpful info. The primary "Do-it-Yourself" upgrades include replacing the ride plate, the intake grate, the impeller, the air filter, sealing the pump, etc.. You can buy a kit that packages a number of the upgrades together or do them one at a time. For $500 to $1000 you can coax another 3-5 mph out of the FX HO.
I'm considering a ride plate and intake grate upgrade for my ski, not for increased speed, but for better rough water handling. I'm on a very busy lake with lots of chop. My 08 FX HO will do 60 mph on the GPS on smooth water, and that is as fast as I want to go. I picked up 1 mph when I switched to 10W-40 synthetic oil (Amsoil).
Last season I only ran regular gas (87 octane). This year I'm at a marina that only sells 89 octane. I can tell no difference in performance. I've never tried premium, but doubt there would be any difference (Premium gas should only be needed to prevent pre-detonation = knocking of the engine. It will give you no more speed. Therefore if the engine is not knocking you are wasting money on premium fuel).