The standard system on an st170 is actually pretty good, you might gain 2-3bhp from a sports cat but then again it'll be much more expensive than a standard one.

I would've thought if the cat brick is clogging the exhaust then you would notice a lack in power right through the rev range. You could always smash the brick out and see what happens. The EML will eventually come on though as the 2nd sensor will soon realise the cat isn't working as it should.