I mean, for the new Kalos Pokemon it is fairly easy to do. If you know the Pokemon's EV's, level, and can guess at the IV's (or take the average from a wide selection of Pokemon) then the base stats can be worked out fairly accurately. But for the Mega Evolutions, how can we know, for example, how high Mega Gengar's Special Attack is. Yes, it is obviously higher than regular Gengar, but how can we know by how much? Unless GameFreak released it at some point and I didn't see it I suppose. But because the form changes only happen during battle it isn't like we can go and check what the stats are when the Pokemon is Mega Evolved.