You should have no problems, even on high and/or ultra. I play on my 2014 iMac (16GB ram, 1TB fusion, i5 2.7, Nvidia 750M), and I can usually have it set on High, some settings on Ultra. My 2013 MacBook Pro Retina (8GB ram, 256GB SSD, i7 2.3, Nvidia 650M) runs almost as well, mostly on high with a few textures set to Medium. There is more lag on the MacBook in certain zones, but it's probably because I have the game installed on an external thunderbolt drive (not enough space on the SSD).
Bosmer Warden, backstabbing Thief and Mischief Maker
“You’re as stealthy as a Mammoth on tip-toes.”
— NPC, The Rift
Just saw this, I run a mid-2015 27" 5k iMac 16GB, with Radeon M290/2GB, and on nearly maxed settings it does 30-60 fps. I lower settings to go into Cyrodil.
The newer ones and Radeon 580 should be considerably better. But: Get more than 8GB RAM, you will not be happy. And while I like the Fusion drive, I'm still running Sierra, it's not supported by APFS, so you might want to look at a big SSD and an external Thunderbolt HDD. If you're already spending the money for the iMac, may as well spend all the money for a good one.