A faster M 4/3 zoom

Not surprised. I wouldn't go above ISO 800 with the G1 and if I planned to crop significantly, I'd put the limit lower than that.

But if you want better low-light capability, I think you should think more about upgrading your body than of upgrading your standard zoom. With the E-M5, I can go to ISO 3200 with roughly the same quality as the one I get with the G1 at 800. And even other, and less expensive bodies, such as the G3, the G5, or the GH2 will make a considerable difference here.