I am having a small dilemma, since last season I have lost 100-200 RPM from WOT. Both engines. MPH is off a bit too. They run and sound fine. The only things I have done is change the oil and gear lube, clean the spark arrestors, and re-loom some of my throttle cables. I also had some very small leaks on the intake manifolds due to the bolts coming loose(went away after re-torque). Leaned out the idle circuit a bit too, was real rich. (this has been done since I noticed the problem) I checked the throttle cables, they are both on the stops with the sticks pegged, still really close in sync. Have run through multilple tanks of different gas so I dont think thats it. I havent changed plugs/wires since I got the boat last spring, maybe a possiblity? Or props messed up? Have accidentally run them in the sand a couple times while idling up to the local sand bar. Any other ideas or things I can look for? I am kind of stumped right now, especially since it is both engines.

I would start looking at the props might have tweaked them in the sand

How many times have you run since spring? You have to take in account air temp, humidity, load etc. Most engine builders will tell you that you hp in summer months. My boat is a few mph faster in low humidity, low temp days. I would also check the amount of ethanol in the fuel.
Also regarding the engines keep track of your compression. As they age and it drops so does hp.

Have run probably 10 times since spring. I usually run non-ethanol fuel, with a few tanks here and there of 93 w/ethanol while traveling. If I run ethanol fuel I always add startron. It has been much more humid this summer compared to last, and hotter, maybe that is whats up. This is only my second year with the boat so I only have last year to compare to.

yes thats a start, did you change fuel filters too, if not try that plus see if the carbs got gummed up. that could of happened over the winter. I lost 200 to 300 rpms with a bad fuel filter back in 1990 when gas was good.

Changed all filters, seems against the odds that both are bad unless I got a bad batch of fuel. I will change them again, cheap and quick to rule out.

They look fine, but that doesnt rule it out.

Don't forget the bottom of your boat. If you keep it in a slip, all it takes is one hot, humid week for the nasty green stuff to grow and it sticks unless you brush it off. That will easily cost you 1 or 2 mph.

Kept on the trailer and cleaned every use. I didnt wax the running surface this year, I dont think that would make any difference though.

Probably more due to the humidity than anything else.