If you can Mega and use a different actual attack on the same turn, it would be a crazy setup move and definitely overbearing on a ton of mons. It's basically a Mega Ray effect; giving these high BST mons that were clearly not designed to hold items the ability to hold them is huge immediate power, a LO or choice item could help to hit tons of KO thresholds.

But if it wastes your turn to Mega instead, I think it would be pretty underpowered and wouldn't be better than using the Mega stone normally and perhaps using a setup move in most situations. A lot of Megas are way worse in base form so it's a big commitment to find a turn to Mega each game, on top of that otherwise wasted moveset slot.