what size centre section and back box will I need for 05 plate sti? I know standard it is 2.5" but are sti's different fittings from normal imprezas? will a 2003 sti fit?

 

Yes

 

a 2003 will fit? is an sti different from a wrx fitment/size wise or are they interchangeable?

 

You can put a full exhaust off any newage turbo on any other newage turbo as long as you dont cross from twin scroll (Hawkeye STi & some Import IIRC) to single scroll.

You have an 05STi by what you first wrote, then yes a centre section and back box will fit off an 03STi.

There is a bore and silencer difference between STi & WRX iirc but a non turbo exhaust could be completely different.

 

what about an 05 non-sti. will that be ok to put on an 05 sti?