"What I find myself wondering with regards to the high pass frequency on the Mid is if the tonal balance changes when comparing the stereo against the mono FM version (besides the loss of spatial dimension). "
Sure. You could take the MID, SBD, and SIDE stems I shared and mix them yourself, see what you think.
I find that when the monitor output of my DAW is set to mono, the SIDE track contributes little to the mix. What I do gain in mono when mixing the MID to the SBD is a wetter sound, and of course, the PA being the inverse of the stage sound, you can hear the louder instruments that the sound guy didn't need to reinforce much, if at all. Like a Marshall stack, for example. September's festival had a two-guy thrash metal band, and there wasn't much guitar in the PA, so this really helps a lot.
The bigger the band, the more spread out they are, the more this technique shines. Singer/songwriters with mics on voice and guitar (do ANY singer-songwriters play anything other than guitar?) don't gain a whole lot except for ambiance and crowd noise.
"I doubt the Mid mic will be close enough to any one source to get any significant proximity effect, especially if its a subcardioid (proximity effect increases with pattern directivity- most significant with a figure-8, non-existent with an omni, and progressively increases for patterns in between)."
Indeed. The point of the high pass on the omni was to reduce pickup from the subs -- at October's Fall Festival there was a single sub, on the ground at center stage, right where I needed to put my mic stand, so the mic array was about 12 feet directly above the sub. The figure-8 would not pick it up as it is in the pattern's null, but that omni right above it may have gotten slammed. Also, the next street over had another stage with a loud band, and without high-passing the omni, the bass rumble from that stage -- as well as from event generators and the city in general -- was quite audible. Directional mics don't stay flat in the farfield like omnis so there is some built-in high-passing going on with a directional. You need to get in close with a directional mic to get the missing bottom end back in, so yeah, you are right -- I may not need any high-passing if I go directional on the Mid mic. If I used a hypercardioid for the Mid then I reckon random bass (a good name for a band) would not be an issue. But with a subcard I still think I'm going to need to use protection. Bottom-end protection. (I leave this for someone with a dirty mind to comment on.)