So, the popular argument among Porschephilles and fans of the latest thing would have you believe that Porsche is only offering the 991 Turbos and GT3s with their PDK gearbox because it is the absolute best-performing transmission they could put in their ultimate performance iterations of the 911, and no performance should be left on the table. On the surface, that's a fairly convincing position, considering that flappy paddle gearboxes do indeed outperform traditional manuals in most every contest of speed.
But it's wrong.
The PDK is not the best-performing transmission they could have used. A CVT is.
A CVT that could handle the torque of the Turbo engine would be the transmission that leaves absolutely zero performance on the table. With a CVT, you never have to shift because you're always in the perfect gear ratio and the engine can always be in the fattest part of its powerband. No wasted energy revving through the weaker parts of the powerband, and zero distraction from having to think about shifting. You just plant your foot and go.
If Porsche were serious about scraping every last bit of performance out of the jar, they would've spent the money they used developing the PDK on coming up with a CVT that can handle more than a Sentra's worth of power. But they didn't.
Why didn't they? Well, I don't work for Porsche, so I can't say for sure, but I suspect it's because the driving experience would be inconsistent with what its customers expect from a sporting vehicle, regardless of ultimate performance numbers. It's just not that much fun to pin the throttle and have the engine moan at a steady RPM as the car accelerates up to speed.
The PDK, then, is a compromise between performance and driving experience. Just like a manual gearbox is these days. It's just the compromise they were willing to accept. As are, apparently, most of their customers.