If you get 587 horsepower out of a brand new engine, how much horsepower would it make once you got a few hours on it? It seems like it would make a little more once things break in a little.

Yes. No.. Maybe. I just thought I'd cover all the possibilities before the Desktop Dyno bunch sits down at the computer with a bag of popcorn and the tv remote,,,,,,,
Now I can add my two cents; engine dyno results are corrected for standard temp and barometric pressure (59 degrees and 29.92) so unless you boat only in 59 degree air at sea level (too cold for me) the max horsepower will be different every time you run the boat.
Yeah, I know; blah-blah-blah,,,,,,,,,,,,

Theoretically you would see a little more power after a motor has broken in but this applys more to stock engines in the 400HP and less range. I'm not sure you will see anything more worth talking about.