If I may jump in here for a minute, it sounds like the best "mod" for your situations, would be the 4.7k resistor "Church mod"...
My reasoning being: a 3-wire battery box is different from a 2-wire battery box, in several ways, one of which being that a 3-wire battery box has TWO input connectors (either mini-plugs or mini XLRs). Thus, for that reason alone, the 3-wire mod would not "work" for your current battery box which you use with other mics.
The 3-wire mod *would* work, but your AT853s would have to have a mini-plug or mini-XLR for each microphone and you would need to get an additional (3-wire) battery box.
Hence, I say, maybe see if Chris Church (Church-Audio) would be willing to do the 4.7k resistor "Church mod"... then, you can still use the same battery box with the AT853s as well as your other mics, AND you can enjoy the benefits of the AT853s being able to take a louder signal with less (or no?) chance of distortion.