I have a 1998 Scarab 26 with a fresh 502 mag with gil marine exhaust and bravo 1 drive, and bravo 26p 4 blade prop.
with 1/2 tank of gas and 2 passengers it revs 4,600 and gps says 65mph in a light chop. (Cruise speed was 30mph at 2900rpm)
The previous Owner gave me a 3 blade quicksilver mirage 25p that i havent tried yet he said time to plane was alot slower with not much more on top end??

Is your WOT Range 4800-5200 with that motor??

I'm not sure its a 1998 502 mag mpi rated at 415hp, according to this its between 4600-5k. But everything ive read on forums says i should run 5k??http://brucebullockmarine.com/mercury/350-454-502.html

Yes to 5k-5100k.

Try the 3 blade. If you can get on plane without blowing out and you don't plan on pulling skiers than the three blade is the one to run. If the boat jumps on plane with the three blade You might want to try a -2 shorty outdrive. 25 pitch should be about right. I would turn the engines 5100 RPM. If you do go to the shorty then you will probably NEED the 4 blade. My 34 SCARAB had 25 Mirage props stock ( 502 magnums - 385 HP ) and turned 5100 RPM. I increased the HP and went to 28 Bravo 4 blades at 5500 RPM. I could easily blow out the 3 blades with stock length bravo drives. I went to the 4 blades and the boat would leap on plane. ( I used to pull skiers ) I then went to IMCO -2 shortys and the 4 blades would slip getting on plane but still got on plane without a problem.

I'd rather not change the drives at this point, just want optimum performance from what i have. If i go down on pitch to bring the rpms up to 5k will i loose top end? or will the added rpms and lower pitch give me more topend?

The one less blade should give you more speed and RPM If you can get on plane properly.

tink,i thought all the 502 mag engines were rated at 415 hp.

Carbed 502mags were 390hp.

According to BBlades calculator my slip % is 13.9 with this setup?