I do have to admit that the Mega Charizard X will be OP indeed. He'll not only have extremely strong physical attacks thanks to his Tough Claws ability, but Dragon and Fire as you said is a near-unresisted STAB combination that appears only to be resisted by Empoleon I think, and also a few other ones I may have missed. Also, Dragon STAB will be walled by any Fairy-types but thankfully as you said it will have other moves to get around them. I know that Fire/Dragon typing has been used already, on Reshiram but it definitely gave Charizard the love he needed.
An Outrage and Flare Blitz coming from this thing is sure gonna hurt.