I am running a VPN over a pair of 1410's and experiencing less than stellar performance ~ about 2Mb/sec. The radios are operating at 54Mb/sec, the signal strength is -32 dBm, the signal to noise is 59dBm, the Ethernet side is running clean.
The only errors that I can see on the wireless seem to be that when under load, the Retries seem to be almost 30% of the Unicast Packets Sent, with about 5% of the retries being multiple retries. The "Protocol Defers" and "Energy Detect Defers" seem to be about 1/3 - 1/2 of the Retries number.
On the VPN side, I have looked at the counters on the ASA5510's, which are dedicated to this link and the counters seem fine. I have adjusted the TCPMSS with the ASA5510 to 1380.
I don not know exactly what is causing the Protocol Defers and Energy Detect Defers. I would imagine that some bandwidth loss could be the overhead from VPN and wireless "turn-around", but my expectation would be closer to 50%.