Well like for like, I've yet to find a cheap MF zoom that can equal a MF prime.....so an extender on a zoom is going to make things worse......so it depends on what results are acceptable to you...my "foto care" huh! 2X extender is rubbish on a Tokina 80-200mm (erm well any pic is better than none I suppose), now my vivitar 70-150 with matching 2X extender, made by Kiron, is acceptable at 300mm..and saves me buying a 300mm prime.
A hand held shot at 300mm, not much depth of field though from about 25 yards away:-
