The 55/1.8 is the sharpest 50'ish lens I've ever shot and the bokeh is smooth and sexy,
I too made the switch only after using my buddies a7rii/55 combo...
My batis 25 is another crazy sharp and just all round awesome lens...I shot/owned about every EF and EF-s mount lens out there from 14mm-400mm...although I never shot the 24 or 85 Art's...
I bought a Metabones 4 to use with all my EF glass...I just didn't think I'd ever be able to let go of my 50 and 85L's (my favorite glass and FL's).
But, the Eye-Focus, sharpness and superior AF of native Sony glass won me over... To me, Sony Glass is, in fact sharper, it really amazes me sometimes when peeping....
Now, it could also be that there's no front or backfocus BS to deal with either...Sheesh, I don't miss that at all....
My 25 Batis, FE55 and 85 Batis are as good or better than the L alternatives, although at 1.8 my 85 doesn't get quite the separation as my L did, but the AF is faster and more "sure".
Then again...if you aren't impressed with the 24-70 GM...Not sure what you are looking for...My GM is really on the same level as my old 24-70LmkII which in itself, is an amazing zoom.
Here's my thoughts summed up I guess...The only ART lens I owned was the 50, and it was an excellent lens for sure..The bokeh on those Sigma primes is so bloody smooth..Even the non ART's have
that smooth mushy bokeh....If you like that, the only FE mount lens that is similar to that (to me) is the 85GM, it reminds me a lot of the Sigma "look"...The 55 is a little different, but does have the sharpness and some really sweet bokeh as well..
Good luck and let us know what you decided on...I'd like to see some shots of the body and lenses with the adapters though...gotta be huge right?
Oh, and I agree, the 5DC was a really amazing body, did portraits like no other ...5DC/85LmkII = Beautiful