Why use Stored Power when LO Clefable with Flamethrower fucks up stall just as badly and is way more useful in general thanks to being able to directly threaten Steel-types. Defensive Mega Venusaur can't beat Clefable if it comes in as Clefable uses Calm Mind anyway, and offensive Mega Venusaur 2HKOes Clefable even after a Calm Mind, while Stored Power is not strong enough just with one boost to do any significant damage back to Mega Venusaur, making Stored Power unnecessary against it. As for Chansey and Quagsire:
- 80 SpA Life Orb Clefable Moonblast vs. 252 HP / 0 SpD Unaware Quagsire: 199-234 (50.5 - 59.3%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
- +6 80 SpA Life Orb Clefable Moonblast vs. 252 HP / 4 SpD Eviolite Chansey: 352-417 (50 - 59.2%) -- guaranteed 2HKO
There, problem solved. With LO and 80 SpA EVs you can even get past SpD Heatran, as +6 Moonblast does 25% minimum. The only things that Stored Power is needed for is to get past Unaware Clefable, which is very easy to wear down as phazing shuts down Wish versions and Moonlight is very easy to stall out for stall teams, and CM Suicune, which is actually the only legit reason to use Stored Power, as stall teams can struggle quite a bit against Suicune.
tl;dr Unless you are Suicune weak don't use Stored Power, and 80 SpA EVs + LO is the stallbreaking CM Clefable variant with the most overall utility.