I have 1999 Colbalt with a 454 Mag EFI that has 300 hours of run time. I've only had this boat for 3 months and the first time I had it out on the water the max RPM"s were 4800. Now after have it out over 8 times the RPM's are down to 4200. No misfires or unusual running of engine. I have replaced the gas/water separator and cleaned the flame arrestor(was pretty dirty). The plugs showed signs of the engine running a bit rich and had some signs of wear. I have a new set of quick burn plugs ready for install.

Is their anything else I can check to rid this problem?

well... at top speed it doesn't take a lot of extra drag to scrub a lot of revs. if it were me, i would put your new plugs in , whatever the hell " quick burn " plugs are... and have a quick look at the tabs and the hull and make sure you haven't fouled the bottom and if all thats correct then you are looking at a missing, probably 50 or 60 hp up top. so look at the throttles... open all the way ? did you change anything between when it was fast and now ?

Do you leave the boat sit in the water? If not, you are probably down a cylinder. Could be a plug, wire, or worse. Sometimes it is hard to hear or feel a miss on a boat.

I lost a cylinder and it did exactly what you describe...didn't sound too bad until you really paid attention. Could be a plug wire...or worse.