Installing the card went without issue. It proved to be a nice, quiet card after installing the nVidia drivers (fans were noisy at first boot -- probably as expected -- until the proper driver was installed).
Interestingly, despite the Fractal Design Arc Midi R2 specs saying it only has 290mm clearance for GPUs with the HDD cage installed, the Gigabyte GTX970 just fit (was a squeeze, but I could get it in). So obviously there is some leeway in Fractal Design's specs for this case.
To test it out I ran through Unigine Heaven again. At 1920x1080 Ultra 0xAA, it scored 2627 (average FPS: 104.3, min FPS: 29.3, max FPS: 205.3). At Ultra 8xAA, it scored 1786 (average FPS: 70.9, min FPS: 8.8, max FPS: 137.6). A little better than the 10 FPS average of the onboard graphics.
Running through Hitman: Absolution in-game benchmark (random game I had available on Steam) it scored Average FPS: 60.2, Min FPS: 50.5, Max FPS: 72.0.