I'm working on a simular setup..except a HP500 now a 509, removed the EFI going to a prosystems blow thru carb...to me seems to be much easier to deal with than the EFI...my .02
I Had Blowthrough On My Last Boat, After Its Dialed In Its Fine, But Getting There Sucks, Id Rather Punch A Few Keys On A Laptop Than Pull Fuel Bowls And Jets.