I've been working with this set and I think I've finally perfected the EVs:
Moveset Name: Mushroom Cloud (Double Powder)
Move 1: Mach Punch
Move 2: Seed Bomb
Move 3: Spore
Move 4: Stun Spore
Item: Toxic Orb
Ability: Poison Heal
Nature: Adamant
EVs: 16 HP / 216 Atk / 232 Def / 44 Spe
Breloom is a pokemon that has a lot of tools at it's disposal that tend to fall by the wayside due to the general effectiveness of the SubPunch Set. Unfortunately in CAP, Fidgit's Vital Spirit and Encore made that set much more difficult to utilize. This Breloom is a toolkit to take on many of the most common CAP Pokemon. It takes a goodly amount of prediction but the ability to cripple or destroy most of the CAP Pokemon makes this variant a keeper.
The EVs on this set are quite unorthodox, but serve their purpose well. HP is set at a number divisible by 8 + 1 so that Breloom picks up an extra point of healing from Poison Heal without taking a point of Stealth Rock or Sandstorm damage. Most of the CAP Pokemon are Physical attackers; of those that aren't, Breloom has little business being in against Syclant and none against Stratagem unless it can snipe them. Fidgit's most common attack is the resisted Earth Power, and Cyclohm should just be statused or avoided entirely. Revenankh is worrisome, however since Shed Skin works at the end of a turn and Breloom outspeeds all the common variants, just Spore until Shed Skin fails and Seed Bomb it, if it comes down to that.
The key to the set is Stun Spore. While Fidgit can switch in on Spore and use Encore to render it useless, if you lure it in and use Stun Spore, the impressive Speed Figdit uses with Encore becomes irrelevant. Seed Bomb is still only a 3HKO, but a paralyzed Fidgit is near worthless. It should be noted that Sludge Bomb Fidgit can counter this Breloom so if you're aware of one, paralyze it and clear out. Stun Spore also has almost near unlimited versatility in crippling many of Breloom's common switchins that treat it like dead weight after it Spores. None of the CAP Pokemon particularly like Stun Spore, and Toxic Orb w/ Poison Heal makes it pointless even for Rebound Collosoil to come in, since Breloom already has Poison status.
Seed Bomb is your main attack, and it can OHKO Colossoil after Life Orb or Spikes damage (90-106%). It also cleanly 2HKO's Arghonaut, and with 44 Spe EVs you outspeed all standard variants. In return, Recover Tank Arghonaut's Ice Punch cannot 2HKO Breloom without weather support. Krillowattaker set takes 82-97% from Seed Bomb.
Mach Punch rounds out the set, doing 62-73% against Stratagem and 45-53% against Colossoil, 33-39% against Syclant, and 21-24% to Krillowattacker (so Seed Bomb + Mach Punch picks up a KO if you need to revenge). With 385 Atk, Breloom's Mach Punch is also an excellent general sniping tool.