The difference will be so small (a few percent) that your users will not notice any change (which is the real factor, not any theoretical overhead). Most likely the wireless link is the slow point in the network, and this will only add a little extra. If you have Cisco (not sure why you want to use dynamic, rather than a conventional IPSEC configuration ?) it is very easy to change the configs to do a VPN tunnel. Then do some tests. If it is a problem, you can change it back in a couple of minutes.
If you hit any connectivity issues, set the MTU on the servers to be around 1300-1400 bytes, which makes sure the overall frame is smaller than the maximum, once the encryption overhead is added.
VPN is a good way to safeguard the traffic over the wireless link, and you are quite right to make sure this is secured.