I knew the 997 had a restrictor in Rolex GT but I thought they let the Mustang go w/out it - it does run one in Koni though - and they have to run 3300 lbs in Koni as well..and still get the sucky brakes. But they are doing better there.
I have not drivena 997 GT3 (I have been in a 996 though) but it seems from driving behind a few - yep, behind, never in front

- that it steps out pretty easy. Not your experience?
Maybe just a factor of the driver, but I have noticed on some televised GT races that they do get loose quite often. That 40/60 weight distribution no doubt comes into play when the tires start to go.