Short word on this. It’s not necessary for filler skills like Acid Purge to deal amazing damage for their use if your cooldown skills are solid enough as is, amazing being something in the ballpark of 120k sheet DPS or above. Radaggan Deceiver gets 5 Sigils that do 800-900 base damage each. Specifically on Acid Purge as well, a big part of it’s damage comes from the 232 base Vitality/Acid damage, the weapon damage makes less difference compared to Flames of Ignaffar that can push double or triple the amount with skill modifiers.
Take a look at some other builds around the forums that use Stormfire, Chain Lightning or Acid Purge as filler skills like Trozan builds for Chain Lightning. They work with 140-170% cast speed and fairly low amounts of flat weapon damage on their filler skills because their Totems/Devils and gear/devotion procs do the bulk of their damage.