I would assume so. It certainly extends to other NMDA receptor antagonists like ketamine and memantine, so it's only logical to figure it might to MXE as well. That's assuming MXE is an NMDA receptor antagonist (which I assume it is). This is something I've been wanting to try for a long time regardless.
Just for the record though, NMDA receptor antagonists don't truly potentiate opioids. They also don't, technically speaking, reduce tolerance once it is already in place (the only thing that truly reduces tolerance is taking a break from whatever drug is in question). What they do is "prevent" (technically speaking) tolerance from increasing/building when taken with your regular dose. A small distinction, one I used to over look, but important nonetheless.
That all being said, I do feel that NMDA antagonists will help speed the decrease in tolerance one gains by taking time off from their DOC, if taking throughout the period that drug is not taken. But this is just my experience, and even then it might not hold. More a feeling I gain from using DXM+piracetam to detox from heroin...