Firstly, welcome to ribnet.
Sounds like you could go down in pitch. the usual rule of thumb is 1" of pitch = 200 rpm. So, based on you aiming at 5500, I'd see if you could borrow a 14" and see what happens.
For what it's worth, my rib is a similar size & HP and I'm running a 14".
I'm now going to go & Google VFI.......