Aligorith has fine tuned his Shapekey transfer tool, clearing up some troublesome problems and speeding up the process considerably. He explains it all on his blog in his usual in depth fashion.
You can test his new version at the same links as last time.
Here they are again in case you need a reminder:
https://sites.google.com/site/aligorith/remap_shapekeys.py?attredirects=0&d=1 <– the script itself.
It’s probably possible to try and register/install this as an addon, but for now, I’d recommend just loading in a text editor and running to get it ready for use.