Well, I got the 8TB IronWolf. I had to spend about 20 minutes shaving off (more like sawing off) a plastic stud in the bottom of my HDD enclosure so that it would fit, and it wouldn't work with my case's screwless design because it's missing the middle HDD holes, so I had to secure it with only two screws in the front. Seems snug enough though...
It seems to be running fine so far but SMART says that ECC Error Rate is FAIL. Everything else is OK. Not sure what that even means. Google doesn't turn up anything. It's a lot faster than my WD Red.
4TB WD Red: 21 read, 62 write, 171 IOPS.
8TB IronWolf: 223 read, 254 write, 7771 IOPS
1TB 840 EVO SSD: 1069 read, 978 write, 83994 IOPS
I threw my SSD in there just for giggles.
I keep most of my games on the WD Red so I'm copying them over to the new IronWolf. I'll swap drive letters once it's done. It should perform a lot better...