Well actually, it depends on your APs - if you've high-end devices processing in HW (seperate CPU), then these will be able to maintain full throughput (ususally). Low-end models perform encryption/decryption via software (running on a single CPU) and these will of course suffer with longer keys. The key (no pun intended) is to implement "whatever meets the requirements". If you have a security issue, then there are other ways of implementing this - e.g. lower the output power of the APs - this may require additional APs, but they could work out cheaper than the high-end kit.
ExCESS
(CWNA)