I have the MRT up-pipe and turbo back and I really like it. It's not to noisy but has a nice rumble to it. I really can't comment on the others, since I don't have them.

As far as boost goes...I researched that same question pretty throughly when I had my boost controller. I was told my many top tuners in the US that a stock WRX is capable of supporting 17psi from the stock turbo (ie: fuel, strength, etc) although it starts to run out of breath around then. I decided on 16psi for my car while the controller was in there.