Please open a support case to make sure you have everything correct. Wireless is half duplex, so you cannot expect the throughout of the associated rate. In addition, things like other clients, aggregation, windowing, scan time, CPU utilization, applications on the host computer that put it off channel will decrease performance. Non 802.11ac devices also play their part in consuming airtime that decreases throughput. Manufacturers have just started writing drivers for AC devices, so that could play the most important factor. Driver optimizations have and will continue to come to increase performance.
Please contact TAC to ensure that you have all of your bases covered.
EDIT: Also use iperf internally rather than rely on external internet speedtests, because you can eliminate things like serialization delays, low window size, etc..