Okay, I've done some poking around. The system actually booted!
So, as you might expect, a 2.4Ghz Atom doesn't offer much in the way of performance. I'm using a Supermicro A1SRi-2758F, which has an Atom C2758.
I have one 8GB RAM stick, two SATA drives, and my Galax GTX 1070 mini connected by a (Molex) powered PCIe x16 to PCIe x8 riser cable. It's all powered by a HDPLEX 160W DC-ATX and a Dell 330W brick.
My Unigine Haven score on my 1070 was almost 30% of what it was running on a slightly undervolted i7-4790K locked at 3.6Ghz, the score was something like ~1400 on the Atom compared to the ~4000's I was getting with the i7. It's not a 1:1 comparison because I was locking the voltage of my 1070 on the i7, but that gives us a rough idea.
Regarding gaming performance, I was honestly impressed. But personally it's not satisfactory for me.
League of Legends is playable, but the (new) client is very hard to navigate, it stutters because of the CPU. (Even on low performance mode.) In game, I can maintain over 60fps but occasionally the game dives, it seems to mostly happen when I open the shop to buy an item... for that reason, it doesn't seem like it would cause problems unless maybe you opened the shop while you were vulnerable and someone jumped you at that moment--you wouldn't have time to react.
Just Cause 3 died the first time I tried loading it. The game booted fine, played its (way-too-long) intro where Rico watches that explosion from the beach, then went to the first main loading screen and crashed. I re-launched the game and it made it through the first main loading screen and warned me I didn't have enough RAM (lol), but let me get into the Main Menu and get the game into fullscreen 1080p with maxxed settings.
The game runs surprisingly well, but I can tell it's not as responsive as it is when I play on the 4790K. It's not bad, though at one point I noticed pretty significant input lag. It almost felt like I was playing Steam in-home Streaming on a cluttered network. Definitely playable, especially for a more casual singleplayer game, but I certainly wouldn't recommend it for someone who was trying to do well or succeed in multiplayer.
All in all, I think it's a route worth considering if power consumption is the utmost importance, but it's also worth considering that this CPU (or rather, the entire system without a GPU) draws about ~30W when it's being stressed. I have heard, but don't have confirmation, that T-series chips, or even drastically undervolted desktop components get better efficiency than Atom-style chips do because as you reach a part's limit, its efficiency goes down. That said, a T-series chip is (IIRC) rated at ~35W and if it's going to normally draw less than that (and idle around ~10W), there might not be much difference in power consumption for the difference in performance. Granted, I don't know how well T-series chips run, I'd assume they are faster.
The other benefit of using a more up-to-date chip is you will almost certainly have a more modern set of features on your motherboard. This thing has USB 3.0, but it is rocking a PCIe 2.0 x8 slot...
And the final thing to consider is that while Atom chips are efficient, this thing sits at ~50C and the heatsink is hot enough that it would burn my finger if I left it on there. It's a "passive" heatsink, but it's intended to be used in a server chassis that has airflow elsewhere. (Like blowing through huge hard drive arrays.)
