I see, now I understand what you're meaning.
I'm not entirely sure where Overcharge fits into the timing in Phase 3. Personally, I'd council against this as Diffusion Chain in P3 is really easy - Ranged/Healers are at max ranged for Whip (staying spread out when Diffused Lightning is incoming... we just stay spread out the whole time except for Static Shock), then strafe out of whip when it's cast, staying at max range, when Thunderstruck is cast then all but 3 people stack in melee then spread after balls. Rinse and repeat
Phase 3 is really easy once you get the rhythm going, it's entirely down to your ranged/healers to be good!
When you say - Keep the Shadow Priest on you so Overcharge always targets him, do you mean during T2?
Levelling Overcharge will make P3 a lot harder than Diffusion Chain imo - that's why almost everyone does SS -> DC