Yeah I'm familiar with the roads your talking about.
It's a very popular spot for drivers of modified cars, whether it be turbo imports or V8 commodores etc. Their all up there regulary. However you really do need to get to know the road pretty well before you gun around their.
People write off cars up their very regulary, if you know the roads and are a good driver then fine, but otherwise be carefull. I personally know a few people who have written cars off up there, then there ones I heard about. As you say in a lot of spots the road surface is pretty average to.
But having said that it's well worth the drive up there, ther are some good roads with reasonable straights and some damn good corners, I've had heaps of fun the 2 times I've been up their. The other thing ther's one road in and out, meaning you can check if the cops are in there.
Unfortunately the gates do get shut at night, 6pm I think (or therabouts).
Paul