turbo back exhaust

i was thinking about buying a turbo back exhaust for my 2004 gti first question would be to go with the 3 inch or a 2.5 inch downpipe and is it better to get the downpipe connected to straight pipe all as one piece or to get thm in two seperate pieces? not really sure if that makes a difference.. Also i guess theres a chance that im going to throw a check engine light with this kind of mod... would upgrading the ecu avoid this problem?
 
Thanks
  
     Mike

is it a 1.8t or a 24v vr6? if its a VR dont go anything past 2.5, id even recommend 2.25 cause the vr needs backpreasure to run correctly. if its a 1.8t then go three, better breathing = better performance in that case. if you unplug your battery when you do the exhaust switch, the Check engine light shouldnt be an issue because the computer should understand the new setup and keep on truckin

yeah if you have the 1.8t, i''d go with a 3" downpipe and 3" exhaust. The only reason you would have check engine lights come on is if you bought an exhaust kit that was for racing (cat-less version)

if you have a stock turbo go 2.5 max, you loose too much low end torque, 3" if modded heavily and chipped only