MacBook – Can a MBA 2014 output 2560×1080 at 60hz

No, it won't support it. The Thunderbolt 1 controller in the Air only supports DisplayPort 1.1 and you need DisplayPort 1.2 to get 4K support, as you mention in your question.

Check out http://www.anandtech.com/show/7049/intel-thunderbolt-2-everything-you-need-to-know and http://en.wikipedia.org/wiki/Thunderbolt_(interface)#Thunderbolt_2

No, this is not possible. As Apple states, the only models capable of supporting 4k resolutions (3850x2160 UHD) at 30 or 60 Hzs are the late 2013 Mac Pros and late 2013 Macbook Pros:

"You can use 4K displays and Ultra HD TVs with your MacBook Pro (Retina, Late 2013) or Mac Pro (Late 2013) computer. You can use these displays at the following resolutions and refresh rates with the built-in HDMI port

3840 x 2160 at 30 Hz refresh rate"

In fact, this is the default configuration.

Though these two models also are capable of supporting 4k @ 60 Hz refresh, multi stream transport must be enabled on the external display.

The same applies to your Thunderbolt port. Thunderbolt 1 is not capable of driving 4K displays. Thunderbolt 2 is.