I had the chance to try the Olympus M.Zuiko Digital ED 40-150mm f/2.8 PRO on OM-D E-M1. This lens is very well balanced on the E-M1, relatively compact for the focal range and fast aperture very well built, and has a very snappy Auto focus.
I pushed it at high ISO , 12800 , 20000 and 25600 …
Here are some sample images with 100% crops at HIGH ISO, jpeg Straight Out Of the Camera.
You can check the photos at higher res at my Flickr Album HERE