A 15X13 would get you up to 4600 or 4700 RPM, and Merc says the 4.3 is good to 4800. But IMHO, I don't like running the 4.3 V-6 that high. It is a 90 degree V-6 that basically is a 350 (5.7L) small-block with 2 cylinders cut off. It is an inherently unbalanced engine that does not like to run high RPMs. The early 4.3 blocks were real buzz machines, but GM went to split bearing journals to smooth it out and that worked OK... Still did not fix the basic problem of a 90 degree V-6.
So my recommendation is to stay with the 15x15 - it will keep the engine in a good RPM range and be fairly efficient. You would get a better hole shot with a 15x13, but it's not clear if your problem is with top speed or hole shot. You did not state what kind of prop it is - aluminum, stainless, or composite - or 3, 4, or 5 blade. You would benefit from switching to a stainless 15x15 if you are running aluminum. Also what speeds are you running at top end? I would guess upper 30s trimmed out - pretty close?
Hope this helps - see ya on the water!
