In terms of the optical design it is indeed very good. It's hard to say it's better than the EF III - it is smaller and lighter and about the same sharpness though the GM has much less vignetting. But lenses wider than 24mm is the area we have seen some benefits to a short flange distance.
But without question it's one of the better GM lenses. However Roger at Lensrentals who sees and tests thousands of lenses and repairs them had to conclude that if one wants a 16-35 GM that is useable at the longer end one would have to test several copies to find that one. I've not seen anything to change that opinion and indeed the same guy has long been adamant that the "later batches improve" theory has no evidence.
I have dabbled with Sony - drove myself mad trying to find a 35/1.4 that was acceptable given its price. There's no question they have some great lens designers, I believe quite a few came over from Nikon. However they still seem to be building even top lenses as if they are consumer electronics - "good enough" quality control and also rarely building it in such a way that allows easy repairs and adjustment later.
In many respects that is the way the whole industry is going. Nikon too has started down that line in terms of repairs and adjustment.
But Canon does remain better and when it comes to things like quality control, build quality, service etc the 16-35L III will be in another league from the GM.
I would love to know what Sony service and support is like with respect to camera equipment. They suck WRT consumer electronics (even their “high end” audio stuff). Canon, on the other hand, is awesome WRT service. You call, they know exactly where your work is and the status, they keep you appraised, etc. This was one of the reasons I decided to stay with Canon, given that I knew that moving to mirrorless would entail upgrading lenses — so now might be a good time to consider a system switch, we’re I so inclined — Canon is so good to work with.