It's been a while, but it seems like when I switched from a 650 double pumper to a 750 DP, I gained .10-.15, which may have been 1-1.5 mph. The 650 DP ran about the same as a 750 vacuum secondary carb. (This was on a 360.) Just my opinion, but unless the engine really needs the air flow or the tune-up is better on the bigger carb, I doubt you'll pick up 2-3 mph.