Few things...
Given that one AP will never really push more than 18Mbps of real throughput anyway (yes, even if all clients are running at 54Mbps), I'd be surprised if you actually have any contention worth worrying about.
Create multiple interfaces on the WLC, and using Group Mappings ("AP Groups VLAN"), you can then assign each AP to a different group. Presuming you set each group to use a different VLAN for a given SSID, and also presuming that each VLAN is configured for a different physical interface, you then get more than enough bandwidth (ie, 100Mbps per AP).
Other option is to use H-REAP, which is a mode of operation that allows your LWAPP AP to behave like an IOS AP, ie, client traffic is dumped stright on to the network without being forced thruogh the controller. This has obvious knock-ons about what the WLC can / can't do w.r.t. monitoring / controlling client traffic.
Regards,
Richard.