Associate
You can fit some nice Nikon and Canon and SIgma fast lenses with the metabones speedbooster and you get faster aperture as well. They become monsters to carry though.
For example a Canon 150-600 FF lens becomes an F4 105mm-420mm lens when mounted on a MFT camera (effective 210 - 840mm). There are some old Olympus fast telephoto lens which become F2 when mounted on the MFT using the metabones.
https://www.camerastuffreview.com/reviews/accessories/
http://www.metabones.com/products/details/MB_SPEF-M43-BT4
Thanks will have a look though the thought of carrying around a huge lense kind of puts me off.
Matt