Found an 18-second long 5120x2160/23.976 clip and dropped it into Premiere, RR enabled... and the clip played very smoothly in both Source and Program monitors at 1/4 and 1/2 resolutions. This supports Wes' recent discovery.
I then exported the clip to DVD-compliant NTSC MPEG-2 (2-pass encode). The process took less than 15 seconds, i.e. faster than realtime (yay!). During export there wasn't much happening in the CPU (2-5% load only, most cores active) but at least the CPU was doing something throughout the export instead of only once in a long while. The main GPU (GTX580) worked at 4%, while the GUI GPU (GTX460) stayed at 0%.
Next test: export to Blu-ray compliant H.264 (2-pass encode at medium bitrate preset). For this test I slapped a Colorista II correction on the clip and added simple film dissolves on both ends. Export took 19 seconds. CPU usage leapt to 60-70% (all cores workout) and main GPU to 35%.