Going with a 3.8 liter means Porsche is leaving themselves room for a hotter 4.0 model down the line. Also, the 3.8 with PDK will be able to match the GT3 RS 4.0 of today likely even exceeding it in acceleration.
The Nurburgring laptime is said to be a very quick 7:34. This means the car would match the much more powerful 997 GT2, impressive. The other information is that rear wheel steering is to be a part of the package and that the debut will be in Geneva next month so it seems the 991 GT3 will debut before the 991 Turbo.
Treat this all as a rumor although it appears to be 100% legitimate.