TC's always lose speed, and sometimes sharpness. A 1.4x will cost you one stop of light and a 2x will be two stops. With a prime lens and a good TC, you shouldn't lose much if any sharpness - it's with zooms that you can have most of the problems. I know Nikon has a TC specially designed to work with their 70-200 2.8 but I haven't used it - I'm sure it's good (but expensive). Minolta may have something like this too.