MacBook Pro Support for 3840 x 1600 (and higher) resolutions
Using these three sites:
Apple display site, Apple Insider site, and MBPr site, I have what I believe to be the correct answer. Using HDMI, you should be able to use a WQHD+ screen at 30Hz. Apple Insider's site recommends the LG 34UM88-P. Unfortunately, it doesn't appear that you computer supports 4K at anything higher than 30Hz. Please let me know if any of the information stated is incorrect. I would recommend finding a store that stocks the monitor you are looking to buy, and asking them if you can bring in your laptop and an HDMI cord to test the monitor and make sure it is compatible with your computer.