The e92 is faster than 996 GT3 in straight line and slower than 997 GT3.
In circuit any GT3 is faster than M3. In a track of 1 km long I go sometimes, the 2007 997.1 GT3 is 3 seconds faster than my 2010 M3 E92 DCT (53.9 vs 56.9 sec). A 2007 997 turbo with some modes (remap, suspension) takes 52 sec, the same as a 2009 GTR with remap and exhaust. After the M3 I think the natural step is the GT3, more fun (RWD) than 997 turbo or GTR, high RPMs like the E92 (9000 RPM) and more lighter.
Next year I am moving from the M3 to a 997 GT3. Here in Chile a 2010 E92 M3 cost $60.000 usd and a 2007 GT3 is about $90.000 usd, the same as a 2007-2008 997 turbo or 2009-2010 GTR (2014 GTR premium cost $140.000 here)