Looks like you're shooting for 312.5 baud, but getting 320 baud? Maybe? Sounds awful slow!
I know you said you read all of the errata and everything...There is a AUSART note in the errata sheets (DS80206F)...
12. Module: AUSART
The AUSART for PIC18F6310/6410/8310/8410 devices may not recognize a received Stop bit if the combined error rate is too high.
Work around
1. Increase the baud rate of the device by decrementing the SPBRGHx:SPBRGx register pair value by one. Verify that the new baud rate does not exceed the maximum combined error
rate of the application.
2. Configure the transmitter to send two Stop bits.
Date Codes that pertain to this issue:
All engineering and production devices.
Try it out...bump it up by one, down by one, whatever works...