From your test results I would suspect your real issue is the WiFi Access Point (AP). The slower bands have this level of throughput. The higher bands allow more throughput but are limited by distance and some walls.
* Review this: [https://en.wikipedia.org/wiki/List_of_WLAN_channels|List of WLAN channels]
You'll need to first check the specs of the unit and look to see how to enable 5.0 GHz set of bands. You may want to label the 5GHz AP slightly different than your 2.4 GHz bands (I append the number 5 & 2 to my AP names). Then within your system set it for the 5.0 GHz band and re-test.
You may also want to get a WiFi scanner to see what channels are being used around you. Ideally you want to use a set that is not used by others or at least channels which the other AP's signal strength is lower so your AP can discriminate it from your AP.
Start there and tell us what you have for a access point and what you get for throughput then.
As to the AC AirPort:
Yes you can swap it out, but the throughput won't be different if you don't have a [https://en.wikipedia.org/wiki/Multi-user_MIMO|MIMO AP unit]. You'll also need to make sure the driver the system is using is the correct one and you have the correct antennas within your system.