rMBP 2015 doesn't recognise Dell P2715Q over DisplayPort
So oddly enough, 10.10.3 didn't seem to make any difference, but the supplemental update that came up yesterday (14D130a) on Software Update (http://9to5mac.com/2015/04/16/apple-releases-os-x-10-10-3-supplemental-update-to-fix-startup-issue/) fixed it right up. Now when I connect the display via Thunderbolt, it comes up at the native 4K resolution at 60hz running in SST mode. Thanks everybody for your help!