If you are just going to run 93 pump gas, with no intercooler upgrade (which the 06 WRX DESPERATELY needs), I would run no more than 17PSI. With an STI TMIC 19PSI sounds like a nice round number, as long as you are tuned for it.
Many people will tell you an MBC is evil and will blow your car up. It can, UNLESS you tune for it.
If you get the MBC, then have the car tuned for the new higher boost level, it will be perfectly safe. Use a GOOD mbc, I highly recommend Hallman.