Not quite the exact same but I did get MU-MIMO to work with 4500c and RFE horns. It was in an omni setup w/4x90deg horns, but it did work. I am pulling it down though due to some things I did not like about that type of setup though.
4500c Omni Setup - Weird Signals and MU-MIMO - Products / ePMP - Cambium Community