have a 1998 502 415 prop hp pushing a 26 ft 5,000lb boat @5000rpm @ 68-69mpf gps , what mph gains would i get if i upgrade engine to make 490 prop hp?

my best guess would be 4 to 5 mph. maybe more with prop work

I'd say 2 to 3 , but I tend to be pessimistic so as not to be too dissappointed .
4 mph might be hoping for too much. The faster you are to start with the more hp you need to gain less. If you can understand that

4-5 MPH is right on the money. If you only get 2-3 your not making 490HP or the prop is wrong. See you on the river.

I don't think that 75 HP would get you 4-5 mph
More like 3.
For an efficient hull 15-17 HP per MPH.
A sonic is not an efficient hull.

I think 4 mph

210HP increase put me up 12 mph in a similar hull.

3-4

I agree....3-4.

zt260 tell me what engine and mods you did to your engine ?