Good news, Gstreamer 1.10.2 works with hardware scaling and glimagesink on the Pi. 1.4.4 is still included by default with Jessie, but using this script you can compile 1.10.2 yourself.
An example pipeline would look like:
gst-launch-1.0 videotestsrc ! capsfilter caps='video/x-raw,width=320,height=240' ! glupload ! glcolorscale ! capsfilter caps='video/x-raw(memory:GLMemory),width=1280,height=720' ! glimagesink
I've still got a few quirks with full 1080p output flickering, but 720p works fine now, and CPU usage is nice and low. I see 15% usage in the above example. As a comparison, video output without scaling is at around 10%. With software scaling CPU usage is 90%+ with dropped frames.