I've lately done many long exposure shots with A7R and lee filters on Canon lenses. Work a treat ! I staked them all the time, big stopper and one 0.6 or 0.9. Result is razor sharp, and WB is a little blueish but easily corrected in raw and Lightroom.
I used screw filter years ago (BW ND1000) and lee filters are really, really better in the field, not even in the same league. Sure it has a cost, you have more stuff to carry and pay care to, but I would not use anything else !
I've set C1 to manual BULB, so I frame and measure light in Av, switch to C1 and put filter holder and shoot with remote. Check this also :
https://itunes.apple.com/fr/app/ndcalc2/id822835884?mt=8
A7R, 24-70II, Lee big-stopper + 0.6, around 90sec exposure
All 2014 shots here done with same setup (some with 70-200II or 17-40) :
Collection Cesure, N.Genette