Mega rayquaza confirmed blah blah blah
Dragon Ascent drops defenses, and is called "one of the strongest flying moves," so 120 bp seems very likely, using that assumption...
Anyways, after watching the video, i ran some calcs:
252+ Atk (180 base) Rayquaza Brave Bird vs. 0 HP / 0 Def Swampert: 301-355 (88.2 - 104.1%) This is a calc shown in the video as an ohko, 170 base attack does not reach this, and so if Dragon ascent is 120 bp as implied rayquaza must gain +30 attack or more.
In the full battle shown, we saw rayquaza started at level 50 and 181 hp. This says it had 4 hp evs, making it most likely all mons had plausible ev spreads.
Abomasnow ice shard (which appears to be 2x super effective) did 66 damage:
level 50 252+ Atk Abomasnow Ice Shard vs. level 50 4 HP / 0 Def (110 base defense) Rayquaza: 56-68 (30.9 - 37.5%)
(56, 56, 60, 60, 60, 60, 60, 62, 62, 62, 62, 66, 66, 66, 66, 68)
Zard-y flare blitz did 33 damage:
level 50 0 Atk Mega Charizard Y Flare Blitz vs. level 50 4 HP / 0 Def (110 base defense) Rayquaza: 33-39 (18.2 - 21.5%)
(33, 33, 33, 33, 34, 34, 35, 35, 36, 36, 36, 36, 37, 37, 38, 39)
There is no way for ray to have 120 or higher base defense, as abomasnow cant do enough damage (unless choice banded or something), and these calcs are possible with base 100 defense, but it would require 252 attack neutral nature abomasnow and a -atk nature on charizard, so to me 110 seems much more likely.
If we have 105 hp (given), 180 attack, and 110 defense, this adds up nicely for rayquaza to be able to maintain symmetry and gain 30 special attack and 20 sdef as well. This would result in a final stat spread of 105/180(+30)/110(+20)/180(+30)/110(+20)/95 (bst 780 (+100)). The lack of speed gain (unfortunately) meshes well with the site description that "Rayquaza is a Pokémon distinguished by its high Attack and Sp. Atk. When it Mega Evolves, almost all of its stats get a boost, including these two, and it gains overwhelming power!" Specifically, that one stat does not get boosted, hence almost all of its stats.