Let me interject with my 2 cents:
1. I think you're over-glorify DSLR, AF actually that great @lowlight.
I owned a series of Canon DSLR + a Nikon DSLR + Sony NEX + Panasonic M43. All 4 camera captures perform good in daylight, but in
ultra-low-light (single candle), the Winner is actually my humble
Panasonic GF6. This was a surprised to me, until I examine the spec further, it turn out a cheapo GF6 can focus down to
-4 EV that non of my Canon or Nikon DSLR can match. So much for DSLR superiority, when it comes to Single shot Low-Light, Panasonic is simply Amazing. Combine Panasonic AF with LCD
touch-screen (manual finger focusing) in lowlight, my AF is nearly 100%
2. Olympus EM1 maybe the best of Olympus, it is still a far cry from the Speed & Reliability of a Panasonic GX8 or Gh4 AF. Panasonic has got the secret sauce of AF algorithm.
3.
FF do not = Fast Focus. Sony A7 is actually a very slow to focus camera. If you want a decent AF with FF, buy Canon 5dmk3, 6D, or Nikon D750, D800
4. True that FF has better Low-Light ISO. This a a fact, no matter how much M43 fans argue otherwise. If this kind of Low-Light shoot is all you do, then it does makes sense to switch.
In summary, I think the AF claim is BULL, but the Noise Claim is TRUE.
Here is an example of my failed gondola attempts. I knew I needed more light above all else so I set to f2.8 and ISO 800. Why did the E-M1 then pick 1/8000 shutter speed.
Why on EARTH would you shoot any
Indoor Photo @iso800! This isn't sunny Florida, indoor setting (DSLR or mirrorless) required a minimum of ISO 3200 with F/2.8 for decent exposure. Shooting @iso800, I can tell you from numerous experience, your photo will be severely under-exposed regardless if you have a FF or M43. This photo is obvious an User Error. Had you taken this photo under FULL AUTO-MODE, you would have a better picture.
By the way, here is a Tread
EM1 very low light, noise filter disabled
This guy was able to shoot a series of action Orthodox Easter night service @lowlight using
ISO3200! When it comes to Low-Light, anything lower than ios3200 is simply not enough.