I have a 5Dc with Sigma 20mm f1.8, 50mm f1.4, 85mm f1.4, 150mm f2.8 and 12-24mm, Canon 20-35mm and 70-300mm.
I also have a G1 with Panasonic 20mm f1.7, 14-42mm, Voigtlander 25mm f0.95, Zuiko 28mm f2.8, 50mm f1.8 and 135mm f3.5, Rokkor 28mm f2.8, 50mm f1.7, 55mm f1.7 and Sigma 50mm f2.8 macro.
My own little tests have convinced me that at low to middling ISO's (I can't say at exactly what ISO the image quality nosedives so I'll stay vague...) and when viewed normally or even closely on screen or in print at image sizes up to A3 once images have been processed to get the best out of them G1 images can easily be lost amongst 5D images and neither I nor people I've roped in can reliably or consistently pick out the G1 images.
The 5Dc is getting old now and so is the G1 but I'd be surprised if later generation MFT cameras are really that far behind the latest 5D images when viewed normally but of course some people may be more picky than me.
IMVHO MFT is ok, image quality wise, for most uses and I'm pretty sure that I personally will only see any significant differences when shooting at the highest ISO, when doing severe crops and printing large or when pixel peeping. Even the dynamic range disadvantage of my G1 is mitigated to an extent by the lovely in view histogram and by processing with modern software.
There are a few irritations with CSC...
In low light and even with everything turned down my G1 shines a torch in my eye is unusable for night time shooting.
Many CSC are limited to a max shutter speed of 1/4000, this matters to me as I like shooting with wide apertures even in good light so ND's are needed... and may need to be on for one shot to keep the shutter speed down and off for the next to get the ISO down and / or the shutter speed up. Maybe I'm grouchy but I got used to being able to shoot at f1.4 and 1/8000 sec without ND's.
Some of the newer CSC don't have ISO 100, they start at something else such as 160 or even 200. For me this would mean increased use of ND's.
Until the tourch in my eye thing can be fixed I don't think I could move fully to a CSC. I'd also love a max shutter speed of 1/8000.