Assuming you could could get AUNetsend/Receive to work with acceptable latency, you would still need to clock both machines together. The reason is that even if the latency starts out okay, without this connection it drifts, and by half an hour the latency can be three seconds: play a note and hear it three seconds later. Not useful.
In other words, AUNetsend/Receive doesn't seem to clock to the ethernet port the way FX-Teleport does on Windows machines; the only digital connection to a Mac Mini without an audio interface on it is the TOSlink (optical S/PDIF) port, so that's what you have to use.
If you have two audio interaces - FireWire or otherwise - you don't need to mess around with TOSlink for sync. But you do also need digital sync, of course - any time you have two digital devices, they have to be clocked togeter. You can use the embedded clock in the digital datastream, but you get lower jitter if you run a separate wired run (either word clock or an S/PDIF or AES/EBU run just being used for clock and not audio). It's not a good idea to use optical cable for sync - the jitter is pretty high. (ADATs used a separate 9-pin connection for sync - they just used the lightpipe for audio.)
You're using the same kind of sync, but my guess is that audio-over-ethernet doesn't have the same jitter issues as standard digital transfers.
Finally, I believe that Digital Performer has an option to clock to ethernet, so you wouldn't need the TOSlink connection with a Mac Mini. However, it doesn't support AUNetsend/Receive, so we're back where we started.
Bottom line, I still haven't been convinced that audio-over-ethernet is a viable solution for Macs yet. But I'm sure it will be sooner than later.