The stock side mount will still flow enough for 270bhp but this will get dangerously hot, which will cause the power to be limited due to ignition pull, even with the oem sidemount you should still get really good torque 260-270lbft and have a huge edge on most k03s 1.8T. The THS is the best Golf mk4 FMIC on the market and should easy give you efficient power across the whole RPM range.
You only up the road from us by the sounds of it? If you pop down to us I will stick it on the dyno £foc and point you in the right direction to make sure the cars running safe... put it this way the last K04-023 AGU we remapped annihilated my mates BMW E92 M3 V8 upto 120mph from a 3rd gear rolling start...

Nick