Can someone please tell me if i could put STI turbo on a rs impreza? or even regular wrx turbo. how much horsepower will it gain? and would it do more performance. let me know.

let's see, you will need:
new crossmember, headers, uppipe, downpipe, turbo, relocate the intake(good luck with MAF) intercooler and maybe you will be able to run max 8psi with good tuning, and shortly after that you might blow your engine up because of the compression rate and internal strength.

you want to turrbo your car, buy the whole engine, you are better off that way, and might even be cheaper too. ej205 engine is pretty cheap.

Essentially, the most power you can make from the 2.5 RS [with a turbo kit] would be approximately equivalent to the stock WRX.

let's see, you will need:
new crossmember, headers, uppipe, downpipe, turbo, relocate the intake(good luck with MAF) intercooler and maybe you will be able to run max 8psi with good tuning, and shortly after that you might blow your engine up because of the compression rate and internal strength.

you want to turrbo your car, buy the whole engine, you are better off that way, and might even be cheaper too. ej205 engine is pretty cheap.

The 02 is not MAF, and a crossmember is not necessary with some fancy pipe work.

6-7 psi is plenty to school basic bolt on WRX's. I remember someone with a Legacy who, at I think 8 psi on a basic kit, made about 270 whp and 280 wtq. The 2.5 is nothing to sneeze at.