I’ve the combination 5rc1 → 4.11.
5rc1 to 4.11 does not change to N modes. So I get 54MBit 5rc1->4.11
and 150MBit 4.11->5rc1 using one chain.
Signal is -65 both directions. Tried all settings I can imagine.
Try to disable the A/G supported and basic rates on the AP and then check if the data rates rises.
Maybe the traffic in one direction isn’t so big to start using N rates?