If you are planning high density with a/b/g clients with 60 clients per ap, you are limited by:
54megs/2/60 = .45 megabits of traffic per client. That is under perfect conditions before you factor in any interference.
If you are planning on a/b/g/n your formula would be
130megs/2/60 = 1.08 megabits per client if they are all 2 stream 802.11a/n clients. This is under perfect conditions and if no a/b/g clients associate to that access point. If you limit it to 30 clients, you can at least get 2.1 megabits/client.
Long story short, planning for 60 users per AP does not give you much room for performance and you should not really plan like that if you expect any measure of performance. It is the RF and the client capability that is limiting this performance, not the CPU
Are you working with a local Aruba resource on this project? If so, they can steer you the right way.