Originally posted by Nephthys"Vader was aware of the Mimban legend about the Kaiburr Crystal, a luminous crimson-colored gem that magnified the Force a thousandfold, and hoped to collect this relic along with the captive Rebels." - The Rise and Fall of Darth Vader
Well Wookieepedia says:"Proportionally according to size and density, the Force's power was magnified a thousandfold"
Vader. Vader was already a threat to Sidious without it. With the crystal he'll be absolute monster. Vader made up his lack of mobility for his world class defense. He had multiple ways to stop force lighting, shields, tanking, light saber. Vaders ability to use the force without moving anything on his body while still using his saber. With the added Crystal, Vader would probably crush Sidious.
Vader was not a threat to Sidious in any era, the latter would have demolished Vader in any era without much of an effort.
Also, that is a legend. While Kaiburr Crystal did enhance Vader's abilities and transformed him in to an individual without shortcomings, I am not sure that he was still good enough to handle Sidious with this augmentation.