I found the Mk. II a huge step up from the Mk. I - the grip is better suited to my hands, the image quality is a bit better, focus speed in all conditions is much better etc. I bought a used 12-40 a couple of months after buying my E-M10 Mk. I and was amazed at how sharp and contrasty a lens it is.
I'd rate the IQ being much better. Not so much in terms of absolute resolution, but everything I found limiting about the E-M1.1 and E-M5.2 in terms of RAW files is noticeably less so on the E-M1.2 now that I've had a chance to really challenge it. It's very much a case of a lot of objectively minor improvements make for a major improvement as a whole.
I'm actually finding the RAW files on par with 24MP APS-C in terms of DR and malleability, which were always my challenges with the 16MP m43 sensor files. I'm sure there's still a noise advantage to APS-C, but since I don't tend to push ISO as much when I have top-notch IBIS available, it's not a major concern of mine (my low-light shooting tends to static subjects, which means IBIS or high ISO, but not necessarily both)