Proof it took him years to recover.
It's explicitly stated, although I forgot where (ask Gideon he's the resident Sidious expert) that once Sidious' spirit left his broken body, it traveled to Korriban and then to Byss where it took him like 6+ years to regain his strength.
In DE Emperor makes transfer to another clone body, while Luke was destroying clones. The moment he entered body, he was fully capable of fighting and defeated Luke. A bit later he summoned Force Storm. It demonstrates that there is no need to recover physically or Force wise.
Possibly because he was dying because of the dark side and not being thrown down a shaft and having his body shattered, or that he's mastered the technique. At any rate, this is irrelevant because it's explicitly stated in the game, on many occasions, that the Hero fought the Emperor's Voice. Add the fact that we know the Emperor to be a sith pureblood, and there's your answer.