Stumbled upon this while looking up something else.
Plagueis was a master of esoteric and unnatural aspects of the Force. With it, he was able to manipulate the essence of life, a power Palpatine believed was directly tied to his inward sight. Plagueis could save others from death and, when his power was applied to the extreme, create new life from the midi-chlorians found in all lifeforms. Plagueis even discovered the ability to retain one's identity in the Force while becoming one with it, but this manner of surviving death did not appeal to him, as he was not concerned with the nonmaterial world.
What I believe is, Obi-Wan was of the opinion that becoming a force ghost would be powerful, because to him, he could train new Jedi, talk to Luke and never be hindered by the Emperor or Vader. However many, including Darth Plagueis, and youself (if I assume correctly), don't believe coming a force ghost makes you uber powerful.
So what you see is true.. from a certain point of view. Obi-Wan did become more powerful, however the same wouldn't be said for Darth Plagueis, had he followed this path to immortality.