Looks like something from Final Fantasy. For real.
Delta Stream sounds kinda awesome, but it's actually useless. Why? First, weather teams are only useful in competitive; the offline game is so easy, you'd never use such an advanced strategy. And secondly, weather teams are not common in Ubers, where MRay will most definitely reside. If it were possible to get Turbulent Winds in OU somehow, it'd be a game-changer (because it'd negate Stealth Rocks). But unless they add a move that can induce this new weather condition, it'll be a novelty and nothing more.
I'd hardly consider the extra resistance it provides to be useless. Now it won't definitely for-sure be one-shotted by Ice Beams. Besides, there's always non-competitive multiplayer.
Delta Stream effectively just makes it a pure Dragon type while getting STAB for Flying moves. Really not that big a deal. Nice, definitely one of the better Mega Abilities and Legendary mon Abilities, but not a gamechanger the way it would be if that weather effect could be produced by a Move, and/or in OU.