If you really have the need for a lot of east-west (probably server) traffic, then VCF (or some type of Fabric solution) is probably a better solution to consider. If the traffic is more north-south, then the oversubscription for a VC is more related to uplink capacity, then VC capacity. Now you can make a VC type fabric with upto 7 x QFX5100s and ensure the total oversubscription is 2:1 but creating something like my [very crude] drawing, but for traffic between any 2 switches the oversubscription could be worse case 12:1, if all 48 input 10GE wanted to talk to interfaces on only one other switch. Therefore we you physically locate your servers within a fabric could well matter.
Hope this helps. I hope the drawing comes through, is readable and makes sense. For the 7 node VC "fabric" all 6 x 40GE ports would need to enabled as VCP interfaces.