This is a really good team, but I think I could make a couple of changes to improve it even more:
Tapu lele seems fine, but in my opinion its slightly redundant to be using a Choice Scarf set when you have Excadrill who is really fast under sand anyway and is generally used on sand as a late-game cleaner, similar to what Scarf Lele achieves. Therefore, I'm proposing to change Tapu Lele to the Choice Specs set, which you can use as a early game breaker to pave the way for an Excadrill cleanup.
Tornadus looks fine, just maybe move the HP EVs into SpA so that Hurricane hits a bit harder. Otherwise, I would suggest to perhaps trial Taunt over Knock Off to assist with Stall, but Knock has its uses in crippling Pokémon such as Chansey by removing it's Eviolite.
Lose Milotic. I think I would be right in saying that in OU Toxapex is a much better choice for a bulky Water-Type, being immune to Toxic and having Regenerator to heal. Also, Milotic's main niche on this team seems to be phasing with Dragon Tail, whereas Toxapex spreads Toxic which is generally more consistent in chipping the opposing team for a sweep. It's up to you on this one :)
Tyranitar should, I believe, be running Chople Berry or Leftovers in addition to being more physically offensive, otherwise it's incredibly passive and almost complete Taunt bait. That said, the Mega might not be a bad idea because it's really tanky so you can run a more offensive set.
Excadrill is fine, just replace Rock Slide for Rapid Spin so you get a little bit extra way to remove entry hazards.
Now, Garchomp. I understand completely where you were coming from with using the Mega, but I have to say that regular Garchomp would be better here due to the increased speed. Perhaps Life Orb would be a better choice?
Final Team w/ Changes