Need help please.

1994 Stratos 284 DC 1994 Ev. Intruder 150. 5 1/2" manual jackplate. I've run a 24 Raker, 5600rpm @ 56MPH and a 25 Renegade at 5500 and 55 MPH. Shouldn't I be getting more speed from an 18' boat with a 150HP? Engine compression is perfect, transom is very solid, no hook in hull, no waterlogged foam. When I first came on this board I was told to set the 4 blade at 3.75 below pad and the 3 blade at 3.50 below pad. At these settings my speed dropped 4 or 5 MPH and I would get blowout in the slightest turns. An oldtimer told me that these Stratos require 5" below pad at least. I kept going lower and lower and finally got 56MPH at 5 1/4" below pad with the 3 blade. At 5 1/2" below pad I got 55 with the 4 blade. My two props were both done by Mark. Anyone have any input? Thanks.