What kind of speeds do you think I can see with a 26' Sonic
weighs 5,000 dry.
Has a 496HO w/ a whipple running 5-6lbs of boost.
Somewhere around 630-660 HP.
Is it reasonable to see 80? Or will it be more around the 77 mark?

I would think that you would be around 80. Now for the bad news....If that 496 still has stock guts in it I would be more concerned about broken pistons than I would speed. You asked about the drive in another thread, well its just my opinion but I would be worried about that too. If you get out on the big lake and start catching some air the drive is not going to like it.
Sorry to be a buzz kill but you asked....Goodluck

My estimate would be 78 MPH.

Yes I had the motor redone the same time the whipple was going on.

What prop would ya suggest?
I see 5 blades more than less break the drives.
what about a 4 blade 30p or do you think that would be to big?

Bad-Intention - my 2002 26' sonic did 75 with 1/2 tank of fuel and me in it on lake george last year.This was with a tired hp500 and a 26 lab from Brett at bblades . he has my vote for props .My new motor will be supercharged with about 685 horsepower and I to will be testing different props to get the most out of my sonic.So to answer your question i would think at least low 80s with 660hp and probably a 28 pitch. Trevor