The XR500 use the same Hw as the Netgear R7800. The bigger memory in XR500 is not used...
The R7800 is able to do download speed of 100+ Mbps over OpenVPN with 256 bit encryption.
I have tried it and succeeded with both Voxel and DD-WRT Firmware. DD-WRT is available for XR500!
So its not a HW limitation in the XR500, its in the firmware and how the code is compiled etc.
Voxel has given optimisation information to Netgear but it seems like they have not cared about it?!
First - I don't have an XR500, but the similar R7800. Since DUMA is an addon to the Netgear Firmware, maybe you can find the information in the Netgear GUI.
For the R7800 you can get the information via: Advanced: Security : Access Control