The good thing about the Bosch ECU in an ST225 is it is very adaptive and will be fine with most Bluefin / Dreamscience etc maps even with varying modifications.
The standard cat back isn't the restrictive part in the system, the downpipe and std cat are, so if you want flow, just replace these. The cat back part is just for sound and style up to about 300bhp.
An intercooler is a must with any uprated map as the std one would struggle trying to keep the temps down when the turbo is pulling more boost.
As said, get yourself over to
www.focusstoc.com as the advice there is top notch IMO.