There are very few Pokemon that are outclassed by their pre-evos (only Dusknoir comes to mind and sometimes Blissey) and i think that by now GF would know very well what evos to give to what Pokemon in order to not make the pre-evo better. It's really not that hard, provided you try even a little, so introducing MegaEvos for this reason would be a lame ass excuse.It depends entirely on the evolution they make for it. Of course it can't get any worse than Luvdisc or Farfetch'd so them outshining there evolution would be rather slim but your missing the point. It's that some pokemon could end up being on par or surpassing there evolution. Which defeats the purpose of an evolution in the first place.