I'll give a rate for your version of the team, then comment on the rates of others, as there's some things being proposed that shouldn't be.
This is a solid Rain team, however, you lack the diversity to power through threats to rain teams, such as Slowking and Cofagrigus. that same lack of diverse typing will also make it hard for your team to get through bulky Water-type Pokemon if Tornadus is ever to fall. Also, Pokemon like Snorlax and Porygon2 can also provide your team with issues provided Virizion is removed. In order to patch up these issues, I suggest dropping Gorebyss and Ambipom for Zapdos and Blastoise. I also suggest changing Tornadus' set, as a Choice user that relies on weather is typically not the best choice user, as getting it in costs a turn of Rain.
Zapdos brings some much-needed Electric STAB to this team, allowing your team to power through Slowking and the other bulky Water-type Pokemon that may provide your team with issues. Zapdos also gains access to Volt Switch, a handy move that can provide your team with some much needed momentum, much like How Mew and U-Turn function. Zapdos also gives your team some security against opposing Tornadus, who would otherwise take advantage of your Rain, and beat you with it. Both Zapdos and Tornadus form a very powerful offensive core in UU that can power through a lot of walls found in the tier. Now, Blastoise may come as a strange suggestion, but with two Flying-type sweepers on the team, and a team that may have to undergo its fair share of switching, you'll want something to deal with Stealth Rock or Spikes if they ever manage to be strewn across your portion of the field. Blastoise makes for a better spinner on this team than Hitmontop, as it can take advantage of the Rain, and has access to Roar, which may save your team from a stray setup sweeper that gets a little too greedy. I also wanted to emphasize a change on Tornadus' set, as there are better offensive sets that do more for the team. The set I'm going to suggest allows Tornadus to set up its own Rain, all while preventing being set up on.
I also recommend you play your Mew differently, by first always leading with Magic Coat, setting Stealth Rock, then setting Rain. This gives your team the edge by having your own hazards up, preventing the opponent from setting up their hazards, and then allowing you to bring the pain. It seems silly, but the onset of a battle does not need to be right to the punch. Give yourself time to properly initiate a sweep, then go balls-to-the-wall. Setting Stealth Rock is much more important than just weakening Pokemon that carry a weakness to Rock-type moves. Believe it or not, but without Stealth Rock, Kingdra cannot beat opposing Snorlax, and with the extra 12.5% damage, it can. Don't overlook a move just because it may delay a match by a single turn.
sets

Zapdos @ Life Orb
Pressure
Timid Nature
4 Def / 252 SpA / 252 Spe
- Thunder
- Volt Switch
- Hidden Power Grass
- Roost

Blastoise @ Leftovers
Rain Dish
Bold Nature
252 HP / 252 Def / 4 SpD
- Scald
- Rapid Spin
- Foresight
- Roar

Tornadus @ Life Orb
Prankster
Naive Nature
4 Atk / 252 SpA / 252 Spe
- Rain Dance
- Taunt
- Superpower
- Hurricane
Now, regarding the previous rates. That defensive Tornadus set is terrible. Tornadus is not a defensive Pokemon, and it can use most of those moves in an offensive manner, such as setting its own rain to take advantage of, and using Taunt to prevent status users or other Hazard layers (shits on Sableye too) from doing what they do best. Tornadus serves the purpose of an offensive Pokemon , as that's what it does best. With access to Superpower, Tornadus can get past things that would typically wall it. In regards to Hitmontop, Blastoise just does its job on this team better, as it reduces a weakness to opposing Tornadus, takes advantage of the rain, and can potentially burn physical threats. Nothing against Hitmontop or whoever suggested it, but there is a time and a place.
Neat looking team. Good luck.