"At this moment Yoda turned, and Dooku gasped. Whether it was the play of the
holomonitors, beaming their views of bleak space and distant battles, or some
other trick of the light, Yoda's face was deeply hidden in the shadows, mottled
black and blue, so that for one terrible instant he looked exactly like Darth
Sidious. Or rather, it was Yoda as he might have been, or could yet become: a
Yoda gone rotten, a Yoda whose awesome powers had been utterly unleashed by his connection to the dark side. In a flash Dooku saw how foolish he had been,
trying to urge the old Master to the dark side. If Yoda ever turned that way,
Sidious himself would be annihilated. The universe had yet to comprehend the kind of evil that a Jedi Knight of nearly nine hundred years could wield."- DR
If he's in his prime AND also mastered the Star Forge. Sidious gets godstomped.