Based on looking at the current Porsche line-up and trying to get my head around their process, this is my speculation on what the GT4 might end up as...
 
- 375hp: It's 100hp over the base Cayman, 100hp less than the 991 GT3, and 35 hp over the GTS... Ideally I'd like a horsepower number of three digits, where the first one is a 4, but I doubt it's in the cards.
 
- 375hp with a more aggressive launch control and maybe an ECU tweak or two would likely get the GT4 from 0-60 in 4.1-4.3 seconds which would put it in the neighborhood of the 991S which comes in at 3.9 - 4.3 depending on transmission. 
 
- The 991 GT3 is about 32% more expensive than a C2S. If a Cayman GT4 is the same % over the Cayman S that gives it a starting price of about 85k USD. If the 32% premium is put on top of the GTS starting price, we get 99k USD. Given the GTS includes some of the drivers' options in its base price of 75k USD I'm guessing the GT4 would start somewhere between the upper-80k and mid-90k range if Porsche deliver on what the car could be. 
 
I've said this a few other places as well, but if the GT4 is a true Porsche engineering project, it could be amazingly good. If it turns out to be a marketing exercise with a big spoiler, some plastic bits, and little substance, I'll be at the front of the line crying foul and asking for my deposit back!
 
Again, all of this is my speculation, nothing more!
 
-nh4.