Olympus EM5 II + Olympus 40-150mm f2.8 + Olympus MC-20 2X teleconverter

This setup is capable of producing sharp images.

My bottom-line 2cents are: If you already have the 40-150mm f2.8 lens, and want additional reach for telephoto, you may consider to add MC-20 teleconverter / extender. If you are primarily looking for a telephoto lens, I personally use the Panasonic Leica 100-400mm lens and love that and is my go-to lens for birding with m4/3Rd gear.