As promised, here is what I have in terms of fitting of different plate sizes for the GGG MPC.
1. BPM Level III+ Standalone, 10" x 12", 1 inch thick, single curve, SAPI cut
As you can see in the photos, a 10" x 12" size plate will fit perfectly into the MPC.
It is a snug fit with no bouncing around in the plate bag, no margins whatsoever.
Front view with plates
Plates in the plate bag seen from the corner opening
No margin from the bottom of the plate to the bottom velcro closure
2. Global Armour Level IV ICW 10” x 12”, non-SAPI swimmer's cut, triple curve, with 10.5” x 12” TAG Banshee Plate Backer
Photos show another example of a snug fit.
Front view with TAG Banshee Plate Backer. As you can see, the MPC can accommodate plates that are not SAPI cut, which I think is a great feature that provides some level of compatibility for plates with different cuts, and if you are not too concerned with the cosmetics of tacticool.
No margin from the bottom of the backer to the bottom velcro closure
View of both plate and backer from the corner opening
Front view with plate and backer
Plate and backer seen from the bottom velcro closure; no margin.
3. Armor Australia Level III+ ICW 9” x 12”, triple curve, with 10.5” x 12” TAG Banshee Plate Backer
While a bit narrower than the plate bag interior (0.5" on each side), the excess 0.5" of the plate backer curves up against the sides of the bag interior and keeps the plate in place.
Seen from the corner opening
No margin from the bottom velcro closure
4. HighCom 4SAS-7 Level IV Standalone 10” x 12”, single curve (nominal 9.5” x 11.5”)
Fairly loose fit, the plate will move around a bit in the bag, as there are 0.5" margins on each side.
Pushing the plate all the way up, this leaves a 1-inch margin between the bottom of the plate and the bottom velcro closure
Not sure whether large ESAPI plates (10.125" x 13.25") would fit as advertised; if it did, the bottom velcro closure would only overlap halfway.
Sides would be a pretty tight fit; anything larger than this would undoubtedly not fit, and anything smaller would move around inside the plate bags.
Initial thoughts on my current set-up indicated in #1, I am very satisfied.
I have this rig for quick donning in the event of an emergency while I am in my accommodations.
In my office, I have an LBT 6094A with my Armor Australia Level III+ ICW plates and the original cumberbund retrofitted with FirstSpear tubes.
Hope this helps!
ETA: There is actually a 0.5" margin with the 10" x 12" at the bottom velcro closure. This would allow a good fit with large ESAPI plates.