Is the retina iMac's GPU powerful enough for its huge display?
This is the article you're looking for: The iMac and its 5K display... as a gaming machine? from ArsTechnica.

Nevertheless R9 M9290X is one of the most powerful mobile GPUs on the planet. That should be pretty powerful on development workflows (I'm a UNIX dev too and do development on GPU, mostly for crunching numbers though).

Edit: 280X was a typo, but I looked to the charts for desktop and mobile parts and 280X and M290X. Frequencies doesn't differ much, but memory bandwidth does. Also texture fill rate is 30% lower, but pixel rates are similar. Processing unit numbers were not directly available, but it looks 20% to 30% slower on real world, not 50%. This doesn't make it smoking hot, we're talking a lot of pixels here, but probably the card wont't be bogged down by the display.

What kind of development work you want to do? Level design? Hi-poly modeling? Engine development?