This seems reasonable and (if it is true) it makes me quite excited/ wary of the other X MegaEvos.
But because, as you rightly mention, we only have a sample size of 2, it's hard to conclude anything at this point I guess. It may be possible that other theories are correct (such as X MEvos boosting physical stats and Y MEvos boosting special stats). And it could just be a coincidence.
But I agree with you that using MEvos to change the "theme" of a pokemon is pretty interesting. There are a lot of pokemon (some already confirmed to have MEvos) that have the potential to use different sets due to their movepools, but are inhibited by their ability/ type/ base stats.
It also brings a lot of variation (potentially). If your opponent has a Charizard for example, it could be "regular", it could be the Y MEvo with a dangerous specially-based Sun set or it could be a DD/ choice band/scarf X MEvo. The same applies to Mewtwo.
Don't you think its a bit early to call this? For all we know, Mega Charizard X could get a monsterous attack and speed boost that compensate for the 50% damage. And we do know that Mega Charizard X is the only dragon (besides reshiram) that can smash fairies in the face with a STAB fire move.
Mega Charizard Y is obviously going to be impactful with drought, but it is not an instant drought like ninetails since as you must "use up" a turn to Mega evolve. And considering it also has SR issues getting a switch in and then using up a turn just to set up the sun obviously has its difficulties.