If there are 8mm available, I would stick them on my car in a heart beat. :lol:
However, this is what the official Porsche/Michelin website says - note comment on shallow tread which I think means 4mm :-
Your car 911 Type 911 (991) / As of 2012 911 GT3
Front : 245/35 ZR 20
Rear : 305/30 ZR 20
The Ultra High Performance Tyre for the 911 GT models, Cayman GT4 and 918 Spyder
Specially engineered for usage on the race track (also approved on public roads)
- Shallow tread profile typical for racing
- Excellent grip on dry surfaces
- Limited grip on wet surfaces due to its main application