This is the most talked about engine for just this reason!
I have an EFI version, and read every post about these blue motors for the past three years.
Long and short of it, 200 hours is a good bench mark for valve replacment. Period!
Compresion test won't tell you dick!
leak down test won't tell you dick!
Visual check is the closest you can go, without buying a spring preasure tester.
If your at 200 hours, change your springs!
I'm sure I'm going to hear a few stories of 300 and even 400 hours engines running fine, I don't care! Down time to me is important. Schit we only get 4 months up here. If you can't aford to do the valves, you shouldn't own the boat!