Hello, I have 3 R510's in my home. 1 for each floor, ceiling mounted and placed central to each floor. None of the clients on our network can ever achieve more than 50Mbps. My PC which is 15ft around a slight corner from the AP got 30 Mbps down and 80Mbps up on an AP to Client speed test.
This one is tough, if this was my install, I would from here, I would switch to a different client, I can see you're on 2.4ghz, so the 40mb/s on a single channel seems about as fast as the standard would see. Plus or minus a few mb/s. Can you disable 2.4ghz on your AP and re-test with the same client?