There is no doubt in my mind that a MSJ would provide a somewhat better grind than the rocky..... but not sure how significant that would be.... other than greater control (than the normal stepped rocky) over grind size.
Re second hand Robur or K10 etc.... I wouldnt hold my breath waiting for one of these to come up second hand at a reasonable price.... They would certainly be excellent grinders.... but are so well built and do such a good job.... who is going to sell one off? (when they upgrade to a ??????).... There is just no reason to get rid of one!!!
Waiting for one of these is like me waiting for a reasonably priced second hand Synesso espresso machine.... we can all dream.... but thats about all we will be doing! ::) ::)