Image quality is worse then you will expect from such lens. Superb is Mamiya A 300mm/f2.8 APO, but this lens will not mount without above mentioned modification and is MF only.
To have some sharpness, you need to close aperture at least to f8-f11.
I can with this lens on XF shoot handheld at 1/800s (about 50% usable shots) and from 1/1250s it's almost reliable.
On tripod Uniqball head with loosed internal ball (kind of gimbal) I can use 1/250s-1/300s.
Few test shots:http://www.fpl.cz/fb/m645_300a.jpghttp://www.fpl.cz/fb/m645_300b.jpghttp://www.fpl.cz/fb/m645_300c.jpg