Hi
At the moment your 6500 linecards can only utilise the 32Gbps shared bus. With a Sup 2 you could purchase a Switch Fabric Module which would then allow your linecards (if they were fabric enabled) to access up to 256Gbps on the switch fabric of a 6500.
So if all your linecards were fabric enabled by installing an SFM you could get much greater performance. if you did buy an SFM you have to insert it into slots 5 & 6 on the 6509 chassis because this is where it makes the connections to the switch fabric.
The Supervisor 720 has an intergrated SFM and this is why it needs to go into slots 5 & 6 in the 6509 chassis.
HTH
Jon