The EOS-M is a tremendous value and meets most of your criteria except for an optical viewfinder. I picked up an M kit with 22/2 lens in the recent B&H $299 sale and am very happy with it. The IQ is even better than I expected and its low light/high ISO performance is very good. I took some ISO3200 shots at a outdoor party over the weekend when it was really dark - I didn't really expect them to be worth using, but with some noise reduction in LR4, they turned out pretty nice.
The RX1, Sony NEX system and Fuji X100s are all terrific options as well, with advantages over the EOS-M, but all cost 3-9 times as much and won't use your Canon EF lenses!
Regarding optical viewfinders, I think the low-end OVF's on the G-series cameras are too small and dim for practical use. Once you are conditioned to a large, bright view finder of a decent DSLR, its hard to settle for an inferior one.
So in terms of high ISO performance between the RX1, Sony's NEX system, Fuji's x100s and Canon's EOS-M which one is known to perform better? Are there other systems worth looking into? My wife is going to be using this a lot more than me probably. But I can see myself using this in a lot of cases. The EOS-M's ability to take EF lenses is very tempting and missed out on that $300 deal.
I used all of these (and many more) except for RX1, order from best to worst:
RX1 - unknown, based on pictures I've seen, similar to Fuji X100s, maybe a touch better?
Fuji X sensor is majestic APS-C sensor, it (almost) rivals my 6D. Ok, let me put it this way: it's equal to, or better than 5D MkII
If you're looking for compactness, I'd go Sony NEX 3N(Nex 6 if you have large budget), or Fuji X20. If IQ is more important and you're ready to pay for it, Fuji X100S, Fuji X-E1, or new and cheaper X-M1.
If you insist on EOS M (I wouldn't), it may be worth ordering this kit from Canada ($350 for EOS M 22mm+flash):http://donsphoto.com/spec_sheet.html?catalog
I currently have Nex 3N and Fuji X-E1 (beside my Canon 6D), after extensive, similar search to yours in finding 'ideal' small camera, . Of course, finding ideal camera is a fools errand, but that didn't stop me, hence wealth of experience