I was kicking this same ideas around for a while. After hearing of a few horror stories of inproperly re-mapped E.C.U.'s I decided against it. Someone on this board sent me this link to an article that did just what you want to do. http://www.azspeed-marine.com/articles-1a.html It sounds great, really an easy thing to do. BUT, if a few guys here on OSO can give you some of their stories, you may think twice about it. If you can get a hold of DirtLawyer, he can tell you of his problems.
My answer to the need for a little more power was finding an HP500EFI. I should have it delivered in a few days. I am not expecting a BIG increase in speed, but it will give me the 85 horsepower that I wanted to get from my 454 Magnum. Also, if you were to sell your 502 and couple that money with what the upgrades would cost, you can probably find a used HP500 for the same money. That way you increase the value of your boat, those Blue motors are worth something at resale.