75 opti on a 20 seafari
 

I have a chance to buy a 2010 optimax 75 at a real good price, if Im going to keep My Old Yeller I was thinking of getting this engine, How do yall think it will preform compared to My old 75hp 4 cly merc? The boat will plane out decent and runs 32mph with the old 75

It's a big difference in weight . . . 375lbs vs. 275lbs. You'll probably see the same performance WOT and better fuel economy. I'd look around for a different engine with more power . . . if you are going to spend a chunk of money.