I can't speak specifically to ISO because, being a former E-M1 user, I'm still gun shy about shooting with high ISO. However, I think your decision to couple the mk ii with the 10-100 12-100 zoom is a good one, and one I am tempted to do myself, but because I already have the 12-40 and 40-150 I probably won't do it. The 40-150 is a fantastic lens, but heavy for backpacking.
Aside from the increased resolution, the biggest surprise for me is that the AF in the mk ii is vastly improved over the original E-M1. With the E-M1 I rarely used AF because I often needed to tweak it with MF anyway and sometimes it would get confused and hunt around for focus while the window needed for a quick shot disappeared. Not so with the mk ii. It is very fast, spot on sharp focus, and I have yet to see it get confused when there was no obvious reason. My shooting habit is changing so that I am now depending on AF which has allowed me to get some shoots I would have otherwise missed, and they are tack sharp.
Like Tony said, there are many small improvements as well. Customizing is easier. The mk ii is a MAJOR improvement over the original E-M1. It is clear that Olympus listens to its customer base and acts on the feedback received.