I think it's encouraging for Ford that Porsche owners are considering the new GT.
I would say that with a 2015 GT, you'd be getting a significant upgrade in tech toys and technology over your 911. You didn't say what year your 911 is, but since it's a 996 it has to be 99-04. So you'll be getting a car that's 10 years newer for about 40K, and I'm sure your 911 will fetch good resale value to the point where you might not have to pay much towards the GT.
From the reviews out there, the Performance Pack appears to be a nice upgrade that delivers superior street performance but still lacks a bit on the track compared to something like a Camaro 1LE. But for 99% of buyers, it will be more than adequate. I would get it just for the upgraded front Brembos alone.
Reviews on the 6 speed auto paddle shifters has been mixed, with some saying it's decent and others saying it's a bit laggy. It's certainly not going to be on par with a VW DCT or Porsche PDK in terms of quick shifts.
Current: 2015 Ford Focus SE
Former: 2007 GT, Satin Silver