I solved this problem with 2 KM Syncs, 2 keyboards and 2 Mice.

Mouse 1 & Keyboard 1 = main pc only ... Have Multiplicity to do Synergy/Barrier like functions.
Mouse2 = plugged into KM Sync 1 that goes to 4x slave boxes
Keyboard2 = plugged into KM sync 2 that goes to all 5 boxes for key broadcasting.

It smoothed out boxing 100% once I had the main mouse off the slave boxes.