How much top end mph will 600's add to a late model 38TG?

5mph

not for sure on a 38' but on a 39' it's a big difference, how fast is a 39' with 525's, 85-87mph, a 39' with 600's runs like 95-96mph and the one from Houston ran over 100mph with some throttle up prpos.

With a strong tail wind.

I would think a 38 with 525's runs in the 85-88 range, am I wrong?

I thought 38's with 500 EFI's where like 82-83 and the 525's where 85-87mph

38's with 500 EFI's are 81-82 ish. With the 525's, you are right about 85-87 ish as an average. My buddy has one that will do 90. I know of a few 39's with 525's that have a hard time hitting 82.

90mph?? With a strong tail wind?

Could have been.