Look, this change in performance numbers is trivial to prove:
Here's the Porsche UK page from March 31, 2019 <
Porsche 718 Cayman - Porsche Great Britain>. It shows the Cayman hitting 62 mph in 5.1 seconds with a 6MT (or 4.7 seconds with a PDK and Sport Chrono).
Today's Porsche UK page <
Porsche 718 Cayman - Porsche Great Britain>. It shows the Cayman hitting 62 mph in 5.3 seconds with a 6MT (or the same 4.7 seconds for a PDK and Sport Chrono).
Let's look at the Cayman S. From March 8 <
Porsche 718 Cayman S - Technical Specs - Porsche Great Britain> we have 0-62 mph as 4.6s with a 6MT, or 4.4 with a PDK (no SC).
Today's Porsche UK page <
Porsche 718 Cayman S - Technical Specs - Porsche Great Britain> shows 0-62 mph as 4.9s with a 6MT, or still 4.4 with a PDK (no SC).
So what does that tell me? It tells me that the 6MT model Caymans with a GPF increase their 0-62 MPH time by 0.2-0.3 seconds over the cars without a GPF.
Why don't the PDK cars take the same hit? Heck if I know! Best guess is that the performance hit isn't equal throughout the RPM range, so maybe the PDK spends longer at lower RPMs where the engine isn't strangled by the GPF restriction as much?