I am actually doing a RMA exchange through T-Mobile because of the proximity sensor issue. I have already tried using WDRT to reflash my T-Mobile branded device and I still have the issue on 14393.1066.
The problem only occurs occasionally, only when my phone has been asleep for a while, and when someone calls me. The touch screen is sometimes laggy at the same time when trying to swipe up to answer the phone. I think there is something wrong with the sensors waking up from a sleep state. I also have the SD card unmounting issue.
I think it's a hardware issue, since I also had a bunch of issues when I tried to update to CU through Release Preview to see if it resolved the problem (it didn't). It seems most people have stable devices on CU (not sure if they are open market or T-Mobile branded).