Tyrantrum is a Serperior Rampardos... bar the lower HP. But no, certain Mega Evolutions have more muscle and function as better wall breakers, without the need t' be choice locked (ohai MegaLuke). Callin' it right meow that Tyrantrum isn't gonna be OU just because it can Head Smash things without recoil. Might end up OU (though I doubt it 'cause Fighting AND Fairy-type weakness), but not just for Rock Head abuse.
... Speakin' of "ite" stones, Eviolite is sort'a the first/prototype Mega Stone... but it works backwards. Mega Evolutions tend to increase attack stats, whereas Eviolite increases defense for NFE Pokes.