I concur with JimRPhoto's post. I too have an Olympus Em1 mark 2. I am thrilled with it.
Fuji makes some of the best lenses in the industry in my opinion. For example, the 16-24 is a great lens. I have never seen any distortion from using this lens. Consult dpreview.com for lenses you might be interested in. Very helpful.
I have an Olympus mc 14 1.4 teleconverter. It should work on your Panasonic lens. It isn't cheap. 350$ I believe. I can't speak for third party products.
Stay entirely away from 42nd Street Photo.
I have used Dodd Camera for twenty years. It is a very good and reputable store.
I found them to be very reputable. I receive offers from them from time to time. I have never had problems with them.