The Mariah Owners Club - Proud new 1995 Shabah Z212 owner looking for speed!

Hello to all! Glad to say that i am a Mariah owner as of last weekend, especially after the time and driving I've spent finding the right one! Took it out for the first time today and MAN! What an awesome boat. They really thought of everything when building these boats. No shortcuts, truly the Cadillac of bowriders. And I have owned two previous to compare with this one and been on countless others. Never found a 21 ft. open bow with EVERY option! But anyway, down to business. I TOPPED OUT AT 40 :( I have some suspicions why and I'd like a little help confirming. Ok first off, I'm running the Shabah Z212 w/ 350 MAG & Alpha One set up. Boat came with 19p aluminum prop and a stingray fin (Bolted to outdrive i guess for planing?). On flat water with two people, full tank of gas and usual equipment load, trimmed all the way down I hit 40 GPS. As soon as i trimmed up to get the bow out of the water just the slightest, the boat would take off to 45 and climbing but I was rev limiting (350 Mag feature 4850 rev limiter) right at 45 mph. I noticed that with trim all the way down, I was running JUST under 4800 r's. Is there slippage or do I just need to get rid of the fin and go with 21p stainless prop and if so, what diameter? I feel like that fin is killing me. It points downward at the tip, just looking like it creates horrible drag. The boat planes out great now, but I've been reading a lot of posts about guys with planing problems with these boats which is so surprising to me. Someone, anyone, help me PLEASE! My last boat did 52 and it was a beat up old maxum with a 305. I know this boat has potential to do better than that, it says so in the manufacturers specs for crying out loud. Did I mention she's running like a dream? Love this boat! Thanks everyone