Is the current channel on 5GHz running at 80 Mhz on the AP? Is your laptop or device you are testing from capable of 802.11AC at 2x2? To get around 400Mbps, you probably need to sit next to the AP, which you are but MR33 isn't really a full blown AC AP. The aggregate from both bands is about 1.3Gbps for the AP. You getting 100Mbps actual throughput seems about ok to me.
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