What do you guys think will be the difference in speed between Twin 454/385 EFI's and twin 502/415 EFI's in a 30 ft boat, Formula 303 SR1 to be exact, there's a difference of 60hp 1 mph? maybe 2 mph?

If both boats are 100% dialed in, then probably 2 mph.

well for 2 mph I guess it really doesn't matter then, if the 454's wind up being a better deal then thats the route I'll go