Just had my twin stock 2000 HP500 Carbs rebuilt by a very credible marine engine builder. During the rebuild they ported and polished the stock heads, switched to ZZ502 cams and rejetted the carbs (fatter on the secondaries I think). Otherwise still stock.

Lost quite a bit on the low end (which was somewhat expected) and midrange AND am running 2-3 mph slower on the top end. Carbs haven't been dialed in since being rejetted and the motors are running pretty rich, but am wondering if I've simply got too much cam. Seems to be mixed thoughts on the ZZ502 cam in a marine application on this board.

Any thoughts would be greatly appreciated as I'm considering going back to the stock HP500 Carb or maybe even the HP500EFI cams.

If it's more likely due to a restriction elsewhere in the engine (heads, exhaust, carb, etc.), please let me know as I do plan to add AFR heads and CMI headers over the winter.

What are the numbers for the ZZ502 cam? I'm thinking that it's a cam for the street, not a marine app.

That cam is not a good choice for marine usage. Do a search here for cams and you will see what you need to be using.