What does a MY03 standard UK STI boost at. Would a 1.5 bar gauge be OK.

I've been searching for what to connect it to. Is it correct it is the pipe that goes into the dump valve or is there a specific plugged outlet like the classic cars.

 

1.5 bar will be ok but why not just fit the 2bar boost gauge , mine peaks at 1.6bar if you see what i mean

 

A tee-piece into the pipe from the manifold to the dump valve, at the dump valve end is the recommended place. You could then run the tube along the intercooler, then through the bulkhead on the drivers side (leave a bit of 'slack' for engine movement)
JohnD