I have driven a 992.2 GT3 but not hard on the track. I have been a passenger in GT3s with talented instructors and they are an amazing car. That said a well set up M3 is surprising competitive at the track. At my last PCA event I ended up passing a lot of GT3s in the A group. The cars that I couldn't keep up with were a couple of well set up 996 turbos.
2015 f80 M3, 2016 GT3