@leephillips - good news is that linux installs fairly painlessly on the iMac 5K. I'm running Fedora 20 and it's going really well. Some slight funkiness with sound but I'm sorting that out now.
Need hardware accel for gnome shell so required this workaround for the ATI drivers: https://bugzilla.redhat.com/show_bug.cgi?id=1054435