Hi and welcome to the Seadoo forum. I'm not sure what marine gas is, at our lake here the gas they sell at most marina's is 90 octane ethanol free. Since my boat stays at the lake this is what I use. If you trailer your ski what you are using is at least equal or maybe even better than the "marine" gas. So performance should be the same.
Honestly I can't imagine that gasoline would make a 5 mph difference, maybe the previous owner was telling a little fib, maybe lake conditions were different, wind, weight ratio, trimmed differently, there are a number of factors. I think my first assumption might be the correct one.
BTW if you are getting 64-65 mph, on a GPS on the GTI 130, don't do a thing. The RXP 260 should hit close to 70 mph, the Govt. has limited the speed on recreational PWC's to 70 mph.
Lou