From what I gather DP 1.2 has enough bandwidth, but just barely. I wonder if Gsync adds overhead?
http://en.wikipedia.org/wiki/DisplayPort#1.2
Effective bandwidth: 17.28 Gbit/sec.
That is: 17.28 / 8 = 2.16 GByte/sec.
Bytes in a 1080p frame: 3 * 1920 * 1080 = 6220800 bytes = 6.22 MByte.
Maximum framerate attainable: 2160MB / 6.22MB = 347 frames/sec.
Bytes in a 1440p frame: 3 * 2560 * 1440 = 11059200 bytes = 11.06 MByte.
Max framerate: 2160MB / 11.06MB = 195 frames/sec.
Bytes in a 2560x1080 frame: 3 * 2560 * 1080 = 8294400 bytes = 8.29MB.
Max framerate: 2160MB / 8.29MB = 260 frames/sec.
Bytes in a 3440x1440 frame: 3 * 3440 * 1440 = 14860800 byts = 14.86MB.
Max framerate attainable: 2160MB / 14.86MB = 145 frames/sec.
So 3440x1440 would be really pushing it.
If there's even a little information sent over the cable besides pure frames, there will not be enough to attain 144Hz. Now most monitors won't have speakers. But if you want to send audio over the cable too, you're not gonna attain 144Hz. Also, even when the spec says you can transport 2.16GB/sec, it doesn't mean there are actually chips on the market that can deal with that max speed. It might be a practical decision to not push it to the limits.