Thunderbolt 3 has twice the bandwidth of Thunderbolt 2—terrific. But it cannot connect to most displays without an adapter of some kind; see Options for Connecting a Display with Mini DisplayPort or DisplayPort input to a Mac with Thunderbolt 3 / USB-C.
There is also the issue of using existing Thunderbolt 2 devices, such as (in my case) five Thunderbolt 2 OWC Thunderbay 4 units containing hard drives. There is no point to upgrading them for performance; the speed won’t change since hard drives don’t come close to maxing-out Thunderbolt 2.
Display connectivity is a particular thorn, because it terminates the Thunderbolt chain. The OWC Envoy Pro EX Thunderbolt 3 SSD is another such case (terminates the chain, as any bus-powered Thunderbolt peripheral must).
In other words, I have two devices that will terminate the Thunderbolt chain, and only 2 ports on the 2017 iMac 5K. This is one strong point in favor of the iMac Pro, which has 4 Thunderbolt 3 ports on two Thunderbolt 3 busses.
How am I going to manage this with the 2017 iMac 5K? Here’s how.
Thunderbolt 3 Port #1
Connect to OWC Thunderbolt 3 Dock, which has its own Mini DisplayPort port; this allows my NEC PA302W wide gamut display to operate.
Pug my main storage, a Thunderbolt 3 version of the OWC Thunderbay 4, into the Dock. It in turn has a Thunderbolt 3 port into which I can plug an Apple Thunderbolt 3 Male to Thunderbolt 2 Female Adapter, then hang my existing Thunderbolt 2 OWC Thunderbay 4 units off that.
Thunderbolt 3 Port #2
This port could serve the OWC Envoy Pro EX Thunderbolt 3 SSD or any other Thunderbolt 3 peripheral with that SSD at the end of the chain.
All of this feels quite limiting, which is why the iMac Pro with its four Thunderbolt 3 ports on two Thunderbolt 3 busses is appealing.
Below, the OWC Thunderbolt 3 Dock has a mini DisplayPort for connecting an external display.
Configuring my 2017 iMac 5K
Below, the storage for home and travel:
The iMac 5K along with the NEC PA302W and the Apple Thunderbolt 3 Male to Thunderbolt 2 Female Adapter.