You'll be fine. That computer will do up to full 4k at 60Hz on OS X or Windows, so that resolution will be no problem.
EDIT:
Just realized you asked specifically about HDMI. That I'm not 100% certain on, although you could just use Thunderbolt, which will definitely work.
[doublepost=1456515343][/doublepost]Never mind, it will work. Just found a post on a different forum with a user that's outputting the same resolution over HDMI on a 2012 rMBP 15. Although I don't see any advantage to using HDMI over TB, especially since you could just daisy chain any additional TB devices through your monitor and just have one cord to plug in.
Correct 4K/60Hz is not supported by HDMI 1.4. You can do it with Thunderbolt or DP. However, 3440x1440 at 60Hz requires far less bandwidth thank 4k/60Hz, so HDMI 1.4 is sufficient.
[doublepost=1456529523][/doublepost]Well OP, you had me searching for 3440x1440 things, and I had been looking for a new monitor anyway... So I ordered one of these:
OK so I got the monitor set up. In both 10.11 and Windows 10 my 2014 MBP 13" it will run full resolution over HDMI at 50Hz and over Thunderbolt at 60Hz. Hope that answers your question!