I have never had a problem with detonation. My marina has non ethenol 92 octain fuel. I always thought of it as a safety feature not spinning high rpm's. But Mr Gadget says higher rpms are easier on the drive. " big prop not lugging drive"

I would put boat in water and see what it will free rev to. If it bangs limited at 4800 ok there you go if it keeps on going than you know the answer. I had this same prob on a cat of mine I went all the way down to a 22p from a 28p and gained only a couple hundred with crazy rooster tail(; it ended up being my cam/timing combo and heads. The motor would only pull what it would. Cam timing moved my trq hp curve to 4800. If you look at your hp and 85mph on a 272 boss u r moving hoss. You are in a very small group who have got that speed out of that boat. I had one and loved it but only saw 63 with a 502 mpi. Some phucker stole it. Miss that boat. Beautiful lines.