What's an MBC?
If you're still at pre-mastering, why not just pan the guitars right and left on the platform...instead of using digi-voodoo on the stereo mixdown with uncertain results? Maybe I don't unnerstand something. I wouldn't necessarily go 100%...just move them apart until the sound like they're in-communication, and not remote...wherever it sounds good. If you're saying that the guitars were already panned 100% R&L before mastering, something wrong happened , I think. I sometimes use stereo expander applications to squeeze a too-wide audio track [like sampled concert piano] into a narrower field by going negative on the % slider. Did you do that on the mixdown or something...by accident?? Cuz the guits on the outside sure don't sound 100% panned.
[I was gonna suggest you compare to a pro mix to get an idea of the squeeze in your track.]
If I use expander on the mixdown, I get the delay and verb levels thrown out of whack....and I lose the fine control available by tweaking individial pairs or single tracks before a stereo mixdown. I'd suggest you get the field-spread done right in the raw mix...and just level up and compress lightly ..if needed...at mastering time. My personal boundary, now, is that if I have to do more than EQ a little, slightly compress, and level up at master, I scrap the mixdown, and go back to the platform to get it right.