I just got a refurbished 2017 imac(apple unwittingly gave me 16gb RAM when I paid for 8!), specs should be in signature. I'm using a Motu 828mk3 (firewire only) with thunderbolt3>thunderbolt to firewire. That's 2 adapters and cost around $80 together. It's been a couple of weeks and interface is working good.
The computer is handling projects far better than my previous 2009 imac 2.8, i7, 7200rpm hdd 8gb ram. I can record at a 32buffer with 44.1k without pop's.
I didn't know if the motu would work and there isn't much positive info on the web regarding firewire into the new thunderbolt3, so I took a chance and figured I could send the adapters back if nothing else. Unless someone here has that interface and chimes in I don't think you'll find out if it will work.
USB 2 is faster than it was, but I was looking at the roundtrip latency of different interfaces being suprised that they haven't made progress like I would expect.
I'm thrilled with my new computer! even though it's a 21.5"