That brings up the question, where do they go from here? The current rumor is the GT3 will go to 4.0 liters plus offer a manual transmission option and the GT3 RS will go to 4.2 liters. That is basically it and what is likely the final evolution of the naturally aspirated flat-6 unless Porsche produces a new architecture which is unlikely. They will likely bump the stroke slightly as they have nowhere else to go.
991 GT3 3.8 bore x stroke (mm): 102.0 x 77.5
991 GT3 RS 4.0 bore x stroke (mm): 102 x 81.5
This may very well be the last hurrah of the NA flat-6. If so, at least it will go out with quite the bang.