A mate of mine has a 2006 WRX and no difference in performance once you're off the line. You'll notice a reduction in traction so launches won't be as good, but 50-70 seems a touch quicker in the GTI because of the extra torque available.
It depends if you wanted to match your current power or exceed it. The GTI is very capable and on the road will match a WRX easy, but the R will smash it into the ground.