Lost 300 RPM's

[pigeon laker]

I'm new to this forum,but I've been reading alot of posts and there seems to be alot of good info here.Now my problem.I have a 99 Merc 115 2+2,last spring when I put it in the water I had lost about 300 - 500 RPM's.I put in new plugs,no help.So I brought it into a local mechanic,he tested the compression,spark and fuel delivery,all were fine,he told me to get a smaller prop.This would solve the problem,but,where have the RPM's gone.Also my hole shot isn't anything like it was.I have cleaned the carbs[took them all apart and soaked],no difference.Seafoam did nothing,but as I was decarbing with the Seafoam when the portable gas can was going empty the RPM's jumped up about 300-500.Is this an indication that I am overfueling?Should I rebuild the fuel pump and then check the bleed system.Is it safe to run the motor this way.I forgot to mention that the bottom plug is always clean and wet,The other 3 are somewhat dark,but seem OK.Thanks in advance for any help.