Older 225 hp Yami on 80's vintage Sceptre
 

Estimated 30 mph top speed seems like either tired outboard, or heavy logged hull correct ?

80's 23's are heavier hulls and have a larger fuel tanks. You'd have to work hard to only get 30mph out of it . . . even with the bottom covered in barnacles dragging buckets and running on 5 cylinders you should still be able hit 30mph. There were different flavors of yamaha 225 (2.6L 225 1989 - 1992) yamaha excel 225 2.6L (86 - 89'ish) and then the large 3.1L displacement versions after 1992 (v76, ox66, etc.)

The old excels were strong engines. Had a friend with a Paramount that would bang 60 with one. Couldn't kill it.

What prop?

I have a 1977 Sceptre and run a 1988 Yamaha V-6 Excel (225hp). Last summer at WOT (5800rpms) I ran 43 mph with a light load.

That's what I figured, something likely wrong......

I agree. You either dropped a cylinder or are caring an external 3000#'s of aqua in the hull. That set up should run 40-42 wot with the the "wrong" prop. Something is amiss.

43mph = 37kts

"estimated"....

I have an '89 Sceptre with a 225 ox-66. With 1000# of people and gear, at 30 mph I'm around 4700 rpms.