"Basically, can any 4/3rd system and lenses get that ?? If so at what cost in money, complexity, etc. "
GX1 with the 20mm F1.7 would be closest based on this.
as to smaller sensors, there are ton of good ones but none are going to give the same shallow DOF as the DSLR.
if you were not worried about budget and can accept a fixed, fast prime lens then the full-frame Sony RX1 would do suberbly!
I totally agree. Get a GX-1 with a fast prime, preferably the PL25 (best m4/3 lens, period), and enjoy !! I've taken nearly noiseless 3200 ISO pictures in NEAR DARKNESS with this combo (WAyyyy less light than in your typical bar), and I don't even bother with RAW. Definitely GX-1, GF5, EM-5 (the first one being by far the best bang for the buck option).
ISO 3200, NO noise reduction whatsoever, jpeg file, the room was almost pitch dark.
3200, some artificial light, zero noise reduction.
If you want to go the NEX route, you'll get even better low light, high ISO performance, but limited lens options, unless you want to use manual lenses (be careful, I just fell into this and it's about as addictive as cocaine, beware, lol).
If you want to stay with a real compact form factor, RX100 for 600$, or if you're stinking rich the new Sony FF compact. The FUJI offerings are pretty superb in low light too, but bigger form factor.