Definitely go drive the car again. If you want to feel power, get into second and cruise along just over 3k rpm... then punch it!
If you don't think it's fast then, then somethings wrong. Especially if you're coming from a GTI.
Both my brother and my good buddy have GTIs so I've driven them on many occasions, and it's such a different experience. You need more time behind the wheel. If you like everything else about the Z, you'll love it once you own it - trust me. Especially if you through a few bolt on mods. The stock sound is much too tame to be enjoyed.
I had test driven the 5.0 mustang before buying my Z and didn't think it was that much faster. Also I had previously owned the 350z in high school, then was carless for 3 years, then bought a Honda CBR600rr (nuts fast), then bought my 370z after college. In no ways is it lacking.
GET A Z!