Just to fill in the details. On a turbocharged vehicle, you need as little back-pressure as possible in order for the turbocharger to work at its maximum efficiency. This means you ideally want an exhaust system that is the largest diameter possible - without being excessively loud. This is why vehicles with the large 3.5" exhaust systems are producing big bhp/torque figures on standard turbocghargers.
The decat pipe increase power/performance by removing the exhaust back-pressure caused by the standard catalytic converotor. An up-graded doen-pipe is a good idea, as it removes the internal restictions of the standard pipework. However, for maximum performance you will always get better results from a complete turbo back 3" system - as the complete pipe diameter is larger (as the decat/downpipe systems have to match up with the small original exhaust system flanges.
Less backpressure will produce higher boost pressures - up to a point. As the system begins to become rstrictive again at higher power levels you will end up generating more heat than horsepower. As the power levels go up it really comes down to the details.
Hope this helps,
WRC Technologies Ltd.