I would definitely do this if I had a skyhook, but at the moment I just lift the bag and drain by hand which is such a pain in the arse I'd never consider dunking it back in again. I get consistent efficiency of about 67% into fermenter. If I knew I could get an extra 10% efficiency into fermenter maybe I'd do it but from what I understand it wouldn't be that much for just a mash out at full volume.
Dunk sparging on the other hand I may give a go if I can find a cheap smaller urn second hand. I currently drain the bag over a 19L Big W pot so I could instead dunk it into a 20L urn at mashout temps.
I dunno, my system works for me at the moment so I'm not that keen on changing it, and if I'm gunna change it it becomes a slippery slope towards 3V!