I have a DMVPN tunnel setup between a 2811 and a 2851 router over a 54 Mbit wireless link with AES-256 encryption.  My latency is great, less then 5ms.  However my throughput over the tunnel is averaging only 4 to 6 Mbits.

Is this normal for the max throughput I should expect to see to a 2811 router over a AES-256 DMVPN tunnel?

Would lowering my encryption level give me better performance?

Would it be worth adding a VPN AIM module instead of just using the built in VPN module?