I think the grip levels are definitely more realistic than Forzas' (i've only played up to Forza 4 though) and the speed and aggression of sliding is way more realistic in GT than Forza 4, but (to get back on topic) i do think the cars in GT6 perhaps have a wee bit too much grip in the rear compared to real life.
I've driven a few GT-R's (R32 and R33) but haven't pushed them to the point of breaking traction in the rear (because they weren't my cars!), but from the videos i've seen/rides i've been on it doesn't seem to be that hard. Whereas in GT you need to up the hp and/or put comfort tyres on.
I have however pushed my 'baby GT-R', my 4WD R32 GTS-4 sedan, to the point of losing traction and have found it almost as easy to get the back out around corners as my rear wheel drive GTS. Which itself is easier to slide than the more powerful R32 GTS-25 in Gran Turismo.
So yeah, safe to say there is a bit more grip in the rear in GT6 than real life, but it's not overly unrealistic.