You should be able to get the full 75Mbps via wireless ... with the right client and a correct configuration.
15-22Mbps for me is a indicator that the client is only connected via the 2.4GHz band and 802.11g.
So make sure that the test client supports 802.11n (and high data rates - i.e. my old IPad supports 802.11n BUT only up to a data rate of 65Mbps) in the 5GHz band and that the AP is configured for 802.11n max thruput.
In the IdentiFi Wireless controller user guide there is an chapter called "Achieving High Throughput with 11n and 11ac Wireless APs" - check whether you've configured your infrastructure like that to get the max. thruput.
The max. that is supported on an AP3705i with 802.11n in 5GHz is a data rate of 300Mbps which should give you a thruput of about ~150Mbps (Page#6 in the datasheet).
http://learn.extremenetworks.com/rs/extreme/images/AP3705i-DS.pdf
-Ron