Also should I get a Sigma or Canon convertor to perform best

Just a quick note. Canon AF teleconverters will only physically fit some lenses, these tend to be the high end L series lens. So I do not think they will fit any independant lens manufacturers products or any of the cheaper Canon lenses.

There is information on this in a Canon lens brochure if you can get one.

If you can stretch to a Canon 100-400mm L series lens, a teleconverter will fit.

Phil