Those 15-20 megabit/sec throughputs you're seeing are fine for 802.11g, which is what (unbeknownst to you) you're actually configured to use here.
The IEEE 802.11n spec requires that you do WPA2 (AES-CCMP) or no security, and it requires that you do QoS (WMM). You've blocked yourself from doing 802.11n by doing TKIP-only and disabling WMM.
Also, by setting your channel width the 20MHz only, you've limited your N data rates to 144.4 mbps max. (Don't get me wrong, using 20MHz-wide channels is the right thing to do from a "good neighbor" perspective in the crowded 2.4GHz band. If you wanted to use Bluetooth on a client machine that was also doing 40MHz-wide 802.11n in the 2.4GHz band, it might not work out very well.)
Also note that the rule of thumb for Wi-Fi TCP throughput—on an app that uses TCP in an optimal manner—is 50-60% of the signaling rate. So even if you made the 3 changes I suggested above, and you truly had a good 300mbps N client machine (2 spacial streams in both directions, support for 40MHz channels in 2.4GHz) within good range of the AP, you shouldn't expect to see performance much greater than 150 megabits/sec (about 18 MebiBytes/sec).
By the way, please tell me that these settings (TKIP only, WMM off, 20MHz-only) were not the factory defaults for this product. That would be really sad if they didn't know better and were accidentally effectively disabling their marquee feature by default.