Here are my thoughts... On a turbo car, getting rid of any exhaust restriction is a good thing. The best answer to your question will depend on how far down the mod track you are. If you've got a downpipe and a cat-back and have cranked the boost up (1.2 bar or greater) you'll feel the power hit a little harder without the cat. This will be due to the mild increase in torque that a free spooling turbo will offer over one that is restricted on the turbine side from back pressure. If you haven't got a downpipe I wouldn't worry about the straight through pipe as the first restriction in the chain robs most of the power.
I personally decided to fit a sports cat to my uprated exhaust system. Main reason was that I could not be bothered to fiddle around with the exhaust when my car was examined for the japanese MOT. The cat allowed me to pass the test no problems....but I will definetly replace it with a straight through pipe when my car will be taken back to europe(after passing all sva/mot/and such). As hipogtr said a healthy gain in low down torque will be felt....power wise maybe 3-5 bhp(?)
Oh....it wil probably bang crackle an pop a bit more too....alll good then!
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.