The GT's have a different style of racing.
The Aston, Corvette and some of the other big cars have 520+hp and eat the smaller cars like the 997 and Maserati (about 450hp) down the straigh but loose time through corners and under brakes. It make great close racing.
as for the 427 this is a FIA event, there will be no way in hell Stephane Ratel will allow those inbred mongrels to run. The 427 is an joke and never should have been able to run. It was Holden money that got it entered. During the season the 420hp 360 ran all over the Monaro when it had the weight balast and restrictors on it. The 24 hour races they removed all of these and still only managed to qualify 2nd to JB in 2002