I would take my 5dmk2 over my 80D when still photography is concerned. To my eyes there is still noticeable clarity with the older 5dmk2. I'll say the IQ from the 80D is a big jump from the older 7dmk1/1dmk3 era.
Even though my 80D has decent high ISO performance the overall IQ I get with the 5dm2 still much better. This is using the same L lenses.
Please note I'm talking about CANON crop sensor. Sony, Nikon and fuji are in a different league still when you compare it to Canon's freshest crop sensor offering.
I've compared my Fuji bodies with my 80d w/ $$ lenses and I'll take the fuji files anyday!!! The way the fuji system captures life is extremely pleasant to my eyes.
Discussing ISO also ties in the overall IQ of a camera. AF performance is also part of the equation but this is sort of a different topic.
For casual photos (friends BBQ parties, birthdays etc) I'd go with a Canon 5dmk3 > 5dmk2, fuji X-T10 or X-E2, 80D as far as IQ is concerned. The fuji older 16mpx bodies I own produce rich depth that my Canon 80D cannot easily achieve.
Take it that I do not own the latest X-T2 fuji body. This is a camera that can easily take on my 5dmk2 for AF speed, 4k video with decent AF tracking and excellent high iso capabilities. The X-T2 does not trump the 80D dual pixel technology but otherwise I feel it substantially out performs my Canon 80D in other aspects.
Kaitanium, the M5 will be a good overall performer and apparently like a mini 80D. If video isn't a major concern with the dual pixel technology I'd spend more for a different mature system. I'd definitely spend more on an X-T2 over M5 simply due to lens quality and selection. So far I'd easily take a fuji crop sensor image of the X-T2 over M5 if your primary goal is still image quality.....this is coming from a loyal canon user.