I support Mega Garchomp for A-
Despite its lower speed (and sometimes physical power) than its normal form, Mega Garchomp is a nightmare for stall. Thanks to its good base 120 special attack, it has no problem dispatching Skarmory and Ferrothorn, whereas normal Garchomp tends to struggle a lot against these (especially if it lacks Fire Blast). It can get past physically defensive Rotom-W with Draco Meteor and cares less about WoW. Garchompite is also an item many people would not expect Garchomp to hold. When paired with Tyranitar or Hippo, Earthquake and Stone Edge can take out many threats Mega Garchomp would otherwise struggle against:
4+ Atk Sand Force Mega Garchomp Earthquake vs. 252 HP / 252+ Def Clefable in Sand: 216-255 (54.8 - 64.7%) -- guaranteed 2HKO after Leftovers recovery
4+ Atk Sand Force Mega Garchomp Earthquake vs. 252 HP / 0 Def Mega Venusaur in Sand: 205-243 (56.3 - 66.7%) -- guaranteed 2HKO after sandstorm damage
4+ Atk Sand Force Mega Garchomp Earthquake vs. 4 HP / 0 Def Mega Gardevoir in Sand: 348-411 (125.1 - 147.8%) -- guaranteed OHKO
4+ Atk Sand Force Mega Garchomp Stone Edge vs. 252 HP / 252+ Def Zapdos in Sand: 262-310 (68.2 - 80.7%) -- 50% chance to OHKO after Stealth Rock
Mega Garchomp is a nightmare for stall If normal Garchomp wasn't bulky enough, Mega Garchomp takes this to the extreme: it has 108/115/95 defenses, which allow it to take a beating even when uninvested.
4 SpA Clefable Moonblast vs. 4 HP / 0 SpD Mega Garchomp: 206-246 (57.5 - 68.7%) -- guaranteed 2HKO
252+ Atk Dragonite Dragon Claw vs. 4 HP / 0 Def Mega Garchomp: 260-308 (72.6 - 86%) -- guaranteed 2HKO
252 Atk Tough Claws Mega Charizard X Dragon Claw vs. 4 HP / 0 Def Mega Garchomp: 308-366 (86 - 102.2%) -- 12.5% chance to OHKO
252+ Atk Choice Band Huge Power Diggersby Earthquake vs. 4 HP / 0 Def Mega Garchomp: 280-331 (78.2 - 92.4%) -- guaranteed 2HKO
So, Mega Garchomp, with its great defenses, huge power and an okayish speed (although not as good as Garchomp's), is in my opinion better than all the other Pokemon in B+ (maybe with the exception of Mega Medicham). Unlike Kabutops, Garchomp does not REQUIRE weather, it only appreciates it (kinda like Manaphy). Mega Garchomp deserves to move up.
I agree with this. I also have a couple things I want to say/add on in response to this as I have been using mega Garchomp a lot recently:
1) (At least for my purposes) a 176 atk / 252 spA / 80 spd naughty spread with the same moves you mentioned is potentially the most threatening. The extra attack is very useful against chansey and clefable who could otherwise could spam softboiled and stall out sand (and ultimately garchomp). In addition with sand up chomp (with 176 atk) can:
Get a guaranteed 2HKO on defensive mega venusaur
OHKO Sylveon 87.5% of the time (and not worry about taking a hyper voice)
Get a guaranteed 2 hit on quagsire w/ eq
OHKO Keldeo after rocks damage
93.8% OHKO on Mega Mawile at -1 (and not worry about taking a play rough)
56.3% chance to OHKO Azumarill w/ rocks ups (Azumarill is guaranteed to at least get taken out by sandstorm)
Without sand up:
97.7% chance to 2HKO 248/0/252 Mew (compared to 16% before)
Can OHKO every Tyranitar set (though admittedly sand will almost always be up here)
Guaranteed OHKO on Mega Aero
99.6% 2HKO on Mega Blastoise
I could go into greater detail about what it no longer outspeeds but it really shouldn't miss the 252 speed.
2) I have one quick nitpick with one of your calcs. Mega garchomp almost always runs a -spD nature so the clefable calc is slightly off.
3) I want to quickly address Mega Chomp's reliance on sand by saying that it still can operate very well outside of sand and should not be kept out of A- due to its need for "support."