I wondered about this as well, since I have an (I think) early 2015 13" and notice some slowness when using the display scaling feature or connecting it to an external 4k monitor.
Ars mentions this:
> It’s also much more feasible to use the scaling modes with current GPUs than it was with 2012’s GPUs, which were prone to dropping frames even at lower resolutions. I noticed no slowdown or jerkiness whatsoever in 1440×900 or 1680×1050 mode on the new Pro—you don’t need to worry about performance, so just pick the mode that treats your eyeballs the best.
Doesn't address 5K, but at least seems promising in terms of the ability of the new Iris chips to drive the display at higher resolutions. Hopefully 5k won't be an issue, and Apple is specifically advertising it as supported.
My gut, however, says that if you want to run 5k without any noticeable slowness, you'll need the 15.