It depends which GT3 we're talking about and the driver in the GT3 as well
I've raced a lot on Porsche and these are fast cars when handled properly. I would bet that on a known track, I'd be faster with a stock old 996 GT3 than with my Cobra that has a solid 100hp extra and a very decent chassis (even if still not setup to my taste).
This may not mean the GT3 is a faster car - it's that I know 911's pretty well and know what their strengths and weaknesses are. On the braking and cornering they're very hard to beat.
There are more good Porsche than Cobra drivers out there
Best,
Luke