I haven't gotten around to hiflowing mine, but I have a friend making 216rwkw on about 16psi with his hiflow. He runs a 57mm compressor wheel - but this 16psi is nowhere near maxed out, he still has factory injectors so that's the retarding factor.
Another friend from memory has the 60mm comp wheel and he ran something like 26psi - ran 12.1 in Adelaide. Unsure what power figures he made.
I wouldn't run much more than 20psi on a 54mm hiflow, but again I haven't had one myself, so I'm really just basing this on what I've seen from friends cars. The only way to truly find out what your turbo can do reliably and efficiently is to have it done on a dyno.