I drive on the Ohio River.Its hardly ever rough just some tsunamis from the barges.I dont run it hard and i get it on plane easy.I dont push the throttle way down while starting.The boat has a 454 right now and does about 56.This is a very heavy boat which is why i want bigger power.

No neither of you can rub me..... But you should buy a lottery ticket if you doubt me. This drive still runs strong and as a matter of fact, I came onto plane today at 81 MPH out of Dana point Ca. for a two hour run. Guess what? The drive once again survived! I did buy a backup last year and have never even had to look at it! My god, if I changed my drive fluid every fifteen hours I would have to change it just about every two weeks!

That's what I did, about every two to three weeks and change oil every three to four. I think you are on a great run of luck or are really easy on it. I hope you continue to have the same.
I have had them last 150 hours and as little as 7. Same conditions. Easy out of he hole always in the water I think it is a high spped thing when they give up 85+ and the prop starts to break lose and give up the top end.

What MPH can I expected out of a 26' Sonic with a 496HO whipple charged 5.5psi 660hp on a bravo X ?

What prop do think I should Spin, I am spinning a bravo1 4 blade now at about 5100 rpms with speeds of 66.7 just on the HO.

Forgot to add the whole top of the mototr is being redone to better suit the whipple.

Is it realistic to expect low 80's?
I've always heard it was 16-19 hp for every 1 mph gain, I would be adding about 240hp which if the formula is right would mean I should be somewhere in between
79-82 mph range. Is this correct

Should be close