You would want to start upgrading things like injectors and fuel systems, computer, etc., at roughly 250whp...you can get away with a bit more but it gets dangerous. Even at 250, you need to be pretty careful.
The old school of thought was that the ECU mapped according to boost levels, however it is now known that the ECU simply uses the AFM to derive the primary fuel/ignition maps, and makes corrections from readings of other sensors. So the "above 15psi...." doesn't really hold...it all depends on how much air is going into the engine, and by derivation, how much power you make.
The CT27 and CT20b are other good options as well, instead of going with the to4b upgrade to the compressor wheel.
Megasquirt is okay, but I'd get the one that has ignition control as well if you are going that route.
You can trick the ECU with piggybacks, dont get me wrong, you just need to know how the changes are going to affect the timing maps as well, and adjust them accordingly.