The stigma that's placed on Apple computer's whether its a MacBook Pro or a iMac is it can't game. Meaning gaming on any of their flagship computers does not operate fluidly. Apple is trying to erase that stigma when it officially releases its macOS HIGH Sierra OS update later this year.
The new 27-inch iMac Pro (2017) is packed with some serious hardware, the fully loaded option grants you a Intel Core i7 770K (7th Gen KabyLake), a Radeon Pro 580 graphic chip (8GBVRAM), 64GB of memory, 2TB SSD which sports a 2,650MB read and 2,100MB write speed. So yes if you were to up the cash for the fully loaded variant you pretty much have an Apple machine that rivals most gaming systems. Big question is can the new iMac handle graphically intense gaming? According to a widely popular YouTuber Dave Lee, it can with a bit of a twist of course.
Both the 21.5-inch and 27-inch iMac CPU and RAM can be upgraded, which the widely known You Tuber Dave Lee has performed, bumping up the RAM totaling 64GB, adding a 2TB PCIe NVMe SSD and combining a external GPU resulting in increased speed and responsiveness to the system. Now he stated in his latest video that upgrading hardware in particularly the RAM can be an expensive proposition. This is why it pays to perform upgrades yourself when dealing with Apple computers, you'll come out cheaply in the end.
Now before I post the benchmark results, I want to add that the YouTuber, Dave Lee manage to load Windows on the iMac, with simplistic ease the device was able to detect his external GPU solution via Thunderbolt 3 ports. Now there were some expected problems when gaming with the macOS, he states that problems will eventually solved once macOS High Sierra is released in the Fall.
According to what I've witnessed in the video, gaming performance wasn't bad at all, considering you have impeccable hardware crammed inside a rather thinly design iMac. Before you go on the watch the uploaded YouTube video, peep out the benchmark results I have posted below this passage.
BenchMark Test Done running macOS Ultra settings at 1440p
Diablo 3: 101 FPS
Starcraft 2: 87 FPS
Heroes of Storm: 73 FPS
Left4Dead 2: 73 FPS
Tomb Raider: 76 FPS
CS:GO: 187 FPS
BenchMark Test running Windows Bootcamp Ultra settings at 1440p
Diablo 3: 118 FPS
Starcraft 2: 104 FPS
Heroes of the Storm: 83 FPS
Overwatch: 72 FPS
Battelfield 1: 65 FPS
PlayerUnknown's Battlegraound: 52 FPS
As you can see gaming on the iMac running Windows Bootcamp clearly displays better benchmark numbers than macOS. So yes it is quite possible to play some resource hungry titles on the 27-inch iMac provided that you go with the fully loaded variant, keep in mind, the price point will be rather expensive.
Credit: Wcc and Dave Lee (YouTube)