Even if it was proven with a spectrascope (or whatever piece of tone-measuring equipment) that they were identical, you'd still have people saying they weren't. Mike's opinion is good enough for me. If I wanted a DMM, I would not hesitate in the slightest to get the XO.
I haven't compared them myself. However, an opinion is exactly this: an opinion. An expert's opinion is rightfully considered to carry more weight, but that doesn't make it a law. Personally, I am inclined to believe that individual differences between one big-box DMM and another, and also between one XO DMM and another (due to component tolerances, calibration or anything else) make the answer to the thread's question quite difficult.