If we disable the sleep proxy "feature" on all the MACs will the errors go away?
No. The issue is with the TC and AE, not the Macs, and you can't disable that feature on them.
If you just want those messages not to appear in the log add the system tunable "net.link.ether.inet.log_arp_movements" and set its value to 0.
I actually think the Suppress ARP messages setting will do it as well. If memory serves I didn't add a manual tunable.
I don't have any Time Capsule or Airport Express. Only Apple TV, 2x MacBook that are here sometimes and probably a few PCs with iTunes installed, if that matters.
But I still get the errors in the pfSense log.