Honestly I thought mkII extractors were worth it at the 12/13/14 level, mainly due to the halved build time, rather than mainly for the increased generation. If you don't assist them with engineers, having the mk II as opposed to the mk I is worth ~ 150*13 ~= 2k resources every time you lose a harvester. I honestly don't think that getting a free upgrade of all existing harvesters when you unlock them would be hugely overpowered though and would reduce micro? Of course, with this logic, the mkIII are in some sense worth less than the mkII's while costing more knowledge, which makes them not really worth unlocking(aka since you only save 75 seconds of build time now, rather than 150). Imo the mkIII's need an extra boost of some sort, either making them 12/14/18? or perhaps giving the mkIII's a hp or shield boost or perhaps cutting the build time by even more and/or reducing the build cost.