DPS Disco priest needs to balance a lot of buffs - both to allies and themselves - to have optimal damage output. However, they also have the tax of not having a crit bonus talent. They need to cast multiple non-damaging spells to receive these short duration buffs, and even then they still can't compete with more simple mage rotations on damage because they're a slave to the GCD. The 'simple' fix would be to give them a crit bonus talent, but I think a more creative approach makes the class flow better and more fun (without making Smite spam the ideal damage output).
So my suggestion is this:
Rename Mental Strength to Speed of Light and add the following effect:
Your Holy Fire (and Chastise?) spells cause your next 1/1/2 instant cast spells to have no GCD.
This allows you to weave in casts of Chastise, Power Word Shield, or Smite... and most importantly, all race-specific priest abilities include an instant cast of some form, which fall under Discipline spells. This enables these spells to benefit from this talent, too.
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Additionally, fix Chastise so it triggers Searing Light when you crit allies.