On my GTS25T I found that the De-CAT pipe allowed the turbo to come on boost a bit earlier, 2700rpm V's 3000rpm.
It never made anymore power, although a slight increase in torque was noted on the dyno.
I do have a Nismo front pipe, so the effect would be more pronounced than with the standard front pipe. it never made a great deal of difference to the noise, but my car is very quiet on the exhaust side of things anyhow.