I went back and read the original post and done some thinking because I am at a loss of what your issue is. Only thing I can come up with is when you quote speed of 56 plus 2 more mph when you trim up is that GPS or speedometer. If speedometer then pay zero attention to the speed because boat speedometers are a highly inaccurate way to measure speed. If that is GPS speed then you truly have a few more mph left to achive what that combo is capable of. When you say 5000 rpm it would be nice if you had another tachometer to verify rpm.
There is only one other thing I can think of and its a long shot...a friend and I were out on his boat one day and it suddenly loss 500-600 rpm on top end. We shut it down and the motor idled smooth. We stop and started the motor and found no issued. Holeshot was perfect so were were at a loss of why he lost rpm. Once back at the ramp we pull the cowl to look at the motor. He had recently changed spark plugs so we wanted to inspect them. We found one of the spark plug wires had came undone. We put it back on and dropped the boat back in the water and it ran like a scalded dog on top end. If someone had told me a boat would run smooth on 5 cylinders I would have thought they were telling a tall tell but I saw it first hand. Only issue he had was losing 5-600 rpm...like I said it a long shot but check your plug wire connections. I am at a loss and will gladly help if I can but I am out of suggestions for now. If your tachometer is correct and you speed is GPS you you have some speed and rpm left on the table. Hopefully you find out what your issue is. I am not giving up, I am just out of suggestions.