Based on what I've seen at the track, the 996 GT3 will not be necessarily faster than the M3 in the hands of a regular driver. It doesn't strike me as a true performance upgrade.
It is a much more interactive car for sure, but personally I would have a hard time convincing myself to spend so much money on something that is not faster.
Why not an Exige Cup, or an Atom, Caterham etc, if only for a track car?
2015 F80 ED
2004 Z4 3.0
2014 X1 28i ED