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Much obliged David, once OS install finishes I'll try prob the second combination (TB+Cat6) and hope to report good news.
CYOA-
It's not ideal. It does force you to keep that first computer on all the time.
But then... if you were to get a Mac Mini you'd have to leave that on all the time too.
The advantage of the TB+Ethernet route is that 1 of the Macs get really stunning performance and the other is pretty damn good.
The advantage of the Mac Mini Ethernet route is that both Macs get pretty damn good performance, and you can power either off at any time and the Mac Mini uses less power 24/7.
I generally like to segregate my storage away from a primary computer, so that I have freedom to change anything later, and so that anything can fail without it impacting others (or at least being cheaply and quickly replaceable). So that lends itself to the Mac Mini route too.
But you can basically prove the idea with the TB+Ether route and then buy the Mac Mini at a later date.
Velocio
Thunderbolt networking is cool, but only between 2 computers. You can daisy chain many drives and monitors, but not many computers... it starts to be an addressing nightmare so it limits what is available.
Wired ethernet is still the way to go here, and I'm not sure what the networking cards are on the iMacs, if gigabit then just make sure your cables are Cat6 and your switch is gigabit and you should be good.
Another way to go would be to mix the above... Thunderbolt to the first computer, ethernet from that computer to the next.
You're not going to be limited on gigabit ethernet, the limitations here are on the performance characteristics of the RAID, and the drives that are in the iMacs. However WiFi and 100Mbps ethernet is likely below the performance of a RAID10 setup and SSDs in the iMacs, hence gigabit ethernet or Thunderbolt is preferred.