So I worked out some Special Attack EVs using some calculations. Please note that this is not with the use of Drought---I'm sure these calculations would be much more impressive if they were. If you use Volcarona on a team utilizing Drought, I highly encourage using a bulkier build. That said, I don't.
Here are the calc's for a Volcarona with 334 Special Attack (This set actually has 336 Special Attack; out of Timid, Modest, and Bold, Modest gave me the best investment for EVs). All attacks used are assumed as Bug Buzz on neutral threats, and Fiery Dance/Bug Buzz when not resisted appropriately:
Volcarona @ Lum Berry/Chesto Berry
Modest
248 HP/150 Defense/112 Speed
- Fiery Dance
- Bug Buzz
- Rest
- Quiver Dance
@ +2 Quiver Dance:
1HKO's 4/0 Garchomp with 1 Spike
1HKO's 252/0 Gliscor with 1 Stealth Rock
1HKO's 4/0 Landorus with 1 Stealth Rock
Chance of 2HKO 252/0 Politoed with 1 Stealth Rock + 1 Spike, or 3 Spike (Note: At +2, 252 Modest Choice Specs Hydro Pump in the rain does not 1HKO Volcarona barring a Critical Hit)
Chance of 2HKO 4/0 Togekiss with 1 Stealth Rock
2HKO's 252/0 Suicune, even after Leftovers
@ +1 Quiver Dance:
1HKO's 4/0 Gengar with 1 Stealth Rock
2HKO's 252/0 Hippowdon with no hazards after Leftovers
2HKO's 252/0 Swampert with no hazards after Leftovers
High chance of 1HKO 4/0 Tornadus/Thunderus with 1 Stealth Rock
(Don't know the %, only like 4 HP difference assuming minimal damage, needs to be at +3 in the rain with Bug Buzz)
Chance at 1HKO 4/0 Infernape with 1 Stealth Rock
@ +0 Quiver Dance:
1HKO 4/0 Excadrill with 1 Stealth Rock (Normally he'd out speed Volcarona and OHKO with Rock Slide, however some do not carry Rock Slide, and they may switch on you assuming you attempt to Quiver Dance when you force a switch)
And something to note, Ferrothorn needs to have more than 252/232 with a beneficial nature in Special Defense to have a chance at surviving a Fiery Dance at +0. Why this is notable is that some Ferrothorn run Thunder Wave, and that could potentially ruin your chance at a sweep. It's rather highly unlikely, butt da moar you no.
EV's are self explanatory for the most part. 248 HP to maximize bulk while allowing it to survive two switches in to Stealth Rock, and help you use Quiver Dance. The reason 112 EVs in Speed is so it hits 264 Speed to out speed 252 Jolly Breloom. This is important, as Volcarona has Chesto Berry/Lum Berry and will be able to switch in on Spore and OHKO with Fiery Dance or Quiver Dance on the switch. If your team doesn't have problems with Breloom, feel free to lower the Speed to something more that tailors to your needs. 262 Speed would have sufficed, but it was bumped to 264 to allow it to hit 396 after Quiver Dance to out speed the base 130's. The rest of EVs were put in to Defense. Modest is to push the Special Attack above 334.
This is the set I'll be using. If I make any modifications to it, in the event it doesn't have enough bulk, I'll post back. But usually you play off of resistances, using Rest to restore health, or take advantage of Volcarona's excellent base Special Defense + Quiver Dance to get your boosts in, so I think this should suffice.
Also. If these numbers prove anything, its that Volcarona needs Stealth Rock/Spikes/Toxic Spikes support. Especially Stealth Rock.