I guess part of my research, before diving to full frame, would be to get a good handle on how the 6D II operates on native glass in low light or againts bright light background scenarios. This is where I find the Sigma's lose me in ability to nail the focus accurately. Even with them calibrated properly (which did help a ton in all other situations) I find I have trouble of them hitting focus on bright backgrounds and low light situations like wedding receptions. Native glass will require me to either use just primes or lose some bokeh ability which isn't ideal for me.
I like the flexibility of a short zoom and the only way to maintain the DoF i'm looking for is full frame and native glass. But it's also an overhaul on my gear which concluded me to ask the question if the 6D II was logical or going to a different setup all together.
I appreciate some of the conversation on ISO and what does what at what ISO...ect... But that conversation seemed to stray from what I was really trying to get some opinions on.
These ART lenses are also super heavy which I have found very fatigueing as long day shoots go on. Also with no IS, it can be hard to lower my shutter speed enough to counteract the balance of the lens on the body. I don't know if I'm talking myself into something else already, but I'm just typing out my general thoughts on what I have.
I use a variety of lenses from Canon L, Canon consumer, Tamron, Tokina, and Sigma. All of them struggle with strong back-light; some more than others. The struggles is not only auto-focus, but also 'washing-out' of the subject.
From FF to APS-c, there just isn't a ton of DOF difference. The exact same framing, with the exact same focal length, at the same aperture will yield almost identical amounts of BG blur on FF and APS-c. Variability comes in when you try to 1.6x crop factor by comparing 24mm on FF to 15mm on crop, changing framing, focal length, or aperture, and so forth. There just isn't a ton of difference to be had.
Lots of folks will debate my second point until the cows come home, all of them, even the dead ones. I have to go to a job interview now, but will try to get some comparison shots up this afternoon.
When all is said, and done, and you've weighed all the options, you will still want a full frame camera. Go buy the best one you can afford. Then you too can expound ad infinitum about the wonders of a 36mm wide sensor.