5K is great an all, if your GPU and connection interface can handle it.
It takes roughly 17.2Gbps of bandwidth to drive a 4K @ 60 fps signal in a single stream (Single Stream Transport); DisplayPort 1.2 has just enough bandwidth to support a single 4K @ 60 fps SST stream, but 5K is far too large for the standard. This iMac comes stock with an R9-M290X(2012 GPU) which supports up to DisplayPort 1.2. To get the bandwidth needed for 5k@60hz on DP1.2, Apple would have to overclock the DisplayPort signal by 50-100% on single stream transport.
It seems like the M295X upgrade is a necessity for this thing to render well.