No. Its based on what ev/ivs/ natures it has. Of course when it is a higher level the stats will be better, but when their levels catch up they will be evened out. Only reason evolving early would give better stats is it would take more battle/exp to grow a level which =more evs, but once they are maxed it won't matter. I use the berries to wipe the random evs at a high level than use proteins, and other roids to raise them as much as those will then ev train.