The wastegate controls the boost by directing exhaust from the engine around the turbine. Overboost is a symptom of a failing wastegate diaphram usually, and can be caused by other problems aswell. The Blow Off Valve's job is to vent pressurized air from the between the turbo outlet and throttle plate when you close the throttle, in an effort to let the turbo spool down smoothly as opposed to an abrupt stop from the pressurized air pushing backwards into the turbo's compressor section.
Like it was mentioned above, the boost gauge hooks-up anywhere that you can get a vaccum signal. Ideally, you'll want to stick it somewhere in the intake plenum. You can also T into the fuel pressure regulator signal line. As long as you stick it somewhere between the throttle plate and the head.
Changing the computer chip can be adequate when running a low boost setup. If you want to push some serious boost, you would be best off with an aftermarket engine management system. However, it sounds like you're just looking for a little extra HP without touching the engine internals. Some people have been successful using nothing more than a rising-rate fuel pressure regulator. Still, it would never be tuned as good as it could. What it comes down to : You'd have to have a chip burned for your particular setup.
As for the injectors, there are two types; Low impedance and High impedance. Performance injectors (most) are low impendence, and OEM injectors are usually of the high impendance variety. If you want to run low impedance injectors, you'll need to add an external resistor pack between the ecu and the injectors otherwise the injector driver circuit in your ECU will fry.
To determine roughly how much HP 8 psi would give you, you'll have to get the specs for you GSR engine. Here are some online turbo calculators that can help you figure stuff like this out :
http://www.turbofast.com.au/javacalc.html
Hope this helps!
