there are different things you can do and in stages to boost your horsepower. Many builders can take the 500 and get it up to 560-580 and not break the bank. you can pretty much make it a 525 by adding aluminum heads as well, just wont have closed cooling. dont be discouraged, you have a great base to build from. contact the following shops.

JC performance
Richie Zul
Precision Marine
Tyler Crockett
Gellner Performance
MDG Performance

All of these guys are stand up boat folks and can offer you many options.

I built up a pair of 500EFI's last winter.....DON'T DO IT. Although we made alot of power and they sound great, I'll never do it again (fully dressed at 630 HP, 700TQ). It would have been alot cheaper to run the engines at 4,500 rpms all the time and re-build them (back to stock condition) after 200 hrs use.

Does the plenum need to be matched with the throttlebody?

Thats pretty impressive 630 hp!! I guess a long trip without refueling was out of the question?

If you put a bigger thottlebody like the 525 Yes you need to open up the stock manifold or you will still be restricted.

We put a Crane 741 series cam in one, reprogrammed the ECU, and did some mild port work on the heads and it made 563 hp. Also added the AZSM throttle body. The motor had valve train issue and had to be refreshened anyways, it wasn't opened up just to make more hp.

Could you give me some prices on a ECU reflash and 2000 cfm throttlebody. Arizona Speed and Marine has a 1300 cfm throttlebody for the 500 but I think yours will hve more airflow

be fair though Russ you made 540's out of yours and went through everything, he is asking about smaller things that can be done and they are plenty. comp changes, throttle bodys are the main, you can change cam and mill the heads for more. mine were 575 hp and not that expensive to do.

I bet that cam sounds BADDASS!!!

i ahve a 525 cam in mine and they sound great, idle like a dream and do what ever I want. they still arent the fastest, but this year I'm going alum heads and am shooting for 625. Mark and tyler know fuel injection like no one else...check with them and weigh out your options....you wont be dissapointed.