Offensive teams benefit from Sticky Web, so it's more of a bulky offense kind of thing.
252 HP/232 Def/24 SDef lets Slowbro avoid the 2HKO from Keldeo's Hydro Pump.
Also, what's the point of Mega Altaria having an Adamant nature if its Attack isn't invested in at all? Here's a better take on bulky Dragon Dance Altaria:
-Dragon Dance
-Return
-Earthquake/Heal Bell/Substitute
-Roost
Nature: Impish
EVs: 248 HP/136 Def/96 SDef/28 Spe
Item: Altarianite
Ability: Natural Cure
The above EV spread avoids the 2HKO from the following: Choice Band Dragonite Earthquake, Mega Heracross Rock Blast, +1 Jolly Charizard X Flare Blitz, Incarnte Thundurus Hidden Power Ice. It also lets Mega Altaria outspeed Mega Lopunny and Mega Manectric after 2 boosts
Looking into it, I can certainly see how your suggested spreads would be better. If Altaria can survive hits from these common threats it can certainly take them out with ease after the DD boosts.
For Bulky offense, You could use Bulk Up Talon.
Talonflame @ Leftovers
Ability: Gale Wings
EVs: 248 HP / 252 SpD / 8 Spe
Careful Nature
- Bulk Up
- Brave Bird
- Roost
- Taunt / Flare Blitz
While I can see where you're coming from, I prefer to run swords dance on Talonflame as it gives it much more sweeping potential. I'll go on Showdown whenever I get the chance and test both these sets out though.