My buddy has a 27' cobalt, wiht a 496 mag (have to double check if HO, I don't think it is) and Bravo 3 drive.

It is his family/fun/performance boat. Tubbing, taking 12 people, and doing poker runs.

Wants to get 10 more MPH out of it.

I'm thinking a blower.

He likes the B3 (and I've driven it many times, the drive does work very well)

Not sure if there is any point of an extenstion box?

On my 20'7" eliminator I raised the drive 3" and gained 10 MPH (older boat, so maybe they mounted drives with lower X dimension than these days)

Would appreciate some input and idea's

Any suggestions?

Have your buddy call us here at Raylar for some of our Mercruiser 496 performance upgrade products and options.
Raylar can probably help get him some more miles per hour and reliable performance from a stock 496.
(866) 496-8181 www.raylarengine.com

Best Regards,
Ray @ Raylar

Adding 10 mph to a Cobalt is going to take more than a Raylar kit or blower.
He should just leave it alone.