I have a complete FF system. I have a complete APS-C system. I have a complete MFT system.
I only shoot with my Fuji APS-C system. My XT1's are pretty much equal in size and weight to my Oly EM1, the differences between the XT1 and EM1 aren't significant. For what I shoot and how I shoot, my XT1's and 1D's are pretty much equal. The differences in IQ between my FF and APS-C isn't significant.
I shoot everything from sports to events to street to landscapes to et al ... with my Fuji's. I haven't shot above ISO 6400 with Fuji, but the 6400 was pretty clean. If extreme low light is your primary consideration, I think the new Sony A7S is your camera.
Wrong. The FOV and focal length is different. A 50mil on APS-C looks and "is" different than a 50mil on FX. The kind of separation and "look" I can get with an 85 1.8 on FX could not be achieved on an APS-C.
Wrong. The kind of separation you get with an 85mm f/1.8 on your FX can be exactly matched with Gary's APS Fuji XT-1 and Fuji's 56mm f/1.2 lens.
Gary said the differences in IQ weren't significant -- not that there were no differences. Interesting that in saying he's wrong you picked an example saying it couldn't be done when in fact there is no difference at all. The 56mm lens on Gary's X-T1 has the exact same FOV as your 85mm on FX and wide open will produce the same DOF as your 85mm will produce wide open.
Yes, there are differences between a FF and APS sensor cameras. And there are differences between mirrorless and SLR cameras. It's not however a case of one is always better than the other.
There are things a mirrorless camera can do that your SLR can't. You value the lower noise you get from your larger sensor when you underexpose it. That's about how you work and what your specific needs are. I traded in my FF gear 1 year and 2 months ago and moved to an APS sensor camera. I don't consider what I did a downgrade. The base ISO on my APS camera is 200; twice, maybe three times, in the past 14 months I may have raised to ISO to 400 but never beyond that. That's about how I work and my specific needs. So a feature of your FF camera that's critical to you means nothing to me. While at the same time I'm delighted with my mirrorless camera and it's ability to use uncompromising wide angle lenses that your SLR can't do.
But Gary is pretty much correct in that the differences aren't really very significant.
Joe