I have a larson Senza Spectre with a 515hp (fresh build) 454 Mercruiser. Has a 4 blade 23p stainless prop Bravo I believe. WOT is 4400rpms. GPS is 52 at most. People are getting 70+ out os this 25' boat. Even Factory specs say 65-68 mph. What is my issue here haha. Prop>?

did you actually dyno the motor or are you guessing its 515? if so at what rpm did make 515? I'm guessing it was not 4400 rpm? If you actually have 515 you should be able to pull that prop much higher than 4400 rpm. you drive height is probably pretty deep and will likely be faster with a 3 blade but you still seem to be way off.

4400 is way too low. In order to get those speeds you need to be up around 5500 more than likely. Try a lower pitch prop and see what it gives you.

drive ratio??

rev limiter ?

I do not know the rev limiter rpms. The gear ratio i believe is 1.50:1 Bravo one. The engine shop built it well, bored 40 over, stroked, blue printed etc with full roller cam lifters and rockers. Huge heads. I should see more than 4400rpms, stock tach by the way.

I may need a 24p prop?

NO, you may need less prop. Your RPM is the problem. Motor either doesn't make the power you think it does OR your are over propped.

Was it dyno'd??? How do you know its 515hp???
What is the timing set at???

+1 with making sure how much power you have, and if you have 515hp then check all the motor i.e. spark plug cables distributor fuel pressure etc

The guy has been building big blocks for 30 years, he builds race car motors mostly. He said mine has roughly 515 to 530hp. It could be the timing, what should it be set at? I have no clue. 8 atdc?