45.3MPH ( 39.3 knots) sounds fast to me, that is probably hull speed on a 5.5m? I am not sure that you will get it to go any faster than that.
What is the rpm range advised by Johnson and whereabouts is 5500RPM within that? Johnson will prescribe a powerband where that 90HP is produced probably 4800-5600rpm at a guess, you want to be propped to be within that range otherwise you won't be getting your 90HP.
I would guess that 5500 is towards the top of the range therefore you could go up in pitch to reduce that a little and you may gain more top end. I believe you lose 250rpm each pitch you go up.
Has it run in yet?
or phone them and speak to Julie.
Have fun with your new toy