From hearing Button after the race, he seemed genuinely suprised at the final pace of the RBR car's. He said that in Q2 where everyone is light they had 3 tenths over the Brawn cars.
I dont think the Brawn cars are going to have a huge difference in fuel loads like 5 laps over the RBR cars. When the weights are released, Im sure it will tell the whole story though.
Im expecting Alonso to fly off the line and be ahead into in the first corner. If Renault have good race pace, having a super quick first stint may just play quite well for them.