I read people getting 60Hz @ 3440x1440 with Dell U3415W 34" Curved monitor, though it does not have USB-C to charge the RMB
and 30Hz @ 3840x2160 with LG 27UD88 27" monitor, but this one has USB-C so the monitor charges while connected and you can use the monitor as a giant dock (one cable to rule them all)
Also check out Viewsonic XG3401, a 34" 3440x1440 curved IPS gaming monitor gaming monitor with USB-C (also matches space gray I think).
I personally would go with either LG 27UD88 or wait for more monitors to come out with USB-C like 27UD88. I believe A 4k monitor with USB-C is a little more future-proof.