Did Darth Maul Become Less Powerful After His Duel With Obi-Wan?

No, he got stronger. The difference between Darth Maul and Darth Vader was never the wounds, because losing limbs did not reduce a Force user's potential. Any drop in power came from the mental failings of the injured, and could be overcome with willpower. Vader never made that recovery. Maul was arrogant enough that losing half his body barely dented his confidence, so his use of the dark side went untouched.

The claim that started the debate#

The idea that Vader never reached his potential because he lost too much living flesh on Mustafar is widespread, and it has a source inside the fiction. The Sith Lady Lumiya, who took extensive prosthetics of her own after crashing her starfighter in a dogfight with Luke Skywalker, claimed that high levels of prosthesis dampened an individual's sensitivity to the Force. She said it was what stopped Vader attaining the full potential of a Sith Lord, and what was blocking her own mastery of the dark side.

Note whose claim it is. Lumiya was a heavily augmented Sith with a personal stake in believing her ceiling was hardware rather than herself. Others, including Darth Plagueis, thought otherwise.

The Legends record comes down on Plagueis's side and treats Lumiya's position as the minority view it is. In spite of the misconception to the contrary, a being that lost one or more limbs was not necessarily reduced in the Force. Any such reduction owed to the mental failings of the wounded Force sensitive, which could be overcome with willpower, and not to a loss of their potential.

The one real physical limit#

There is exactly one exception, and it is narrow.

Force powers like lightning had to be channelled through living flesh, and even then only the possible points of origin were restricted. That is why Vader does not throw lightning. It is not that he cannot generate the power, it is that the hands are not his.

Everything else was still on the table. Even after Vader lost a great deal of his organic body, his cells continued to teem with midi-chlorians. He could overcome any apparent loss of Force ability, with the lightning exception, if he could overcome his mental limitations.

That last clause is the whole ballgame.

Palpatine's own diagnosis#

The best evidence that the block was psychological comes from the person with the most reason to care.

Sidious had waited decades for Anakin Skywalker and was robbed of his perfect apprentice when Skywalker's power was severely diminished by his injuries on Mustafar. He did not conclude that Vader was permanently capped. He believed Vader might still be capable of achieving his full potential with a complete psychological recovery, while keeping an eye out for a Force sensitive who could replace his hobbled disciple.

Both attempts at a replacement failed. Galen Marek chose to sacrifice himself for the Rebel Alliance, and Luke Skywalker put his effort into redeeming his father instead of killing and replacing him. Sidious also knew finding another apprentice as powerful as Vader was extremely unlikely.

So the Emperor's own reading of Vader is that the ceiling was in his head, and that if he ever got past it he would be everything he was supposed to be. He never did.

Maul had the opposite problem#

Maul's injury was, if anything, worse. Obi-Wan Kenobi cut him in half on Naboo and everyone involved assumed that was the end of it.

What followed is one of the strangest survival stories in the setting. He spent years on Lotho Minor in near-madness, and fashioned a crude substitute lower half out of scrap, a prosthetic that resembled an arachnid more than any part of a Dathomirian. His brother Savage Opress eventually found him and took him back to Dathomir, where Mother Talzin built him proper cybernetic legs. Later, Death Watch under Pre Vizsla gave him mechanical legs that restored him to his original height.

The point is what did not happen. There is no period where Maul is diminished in the Force. He came back and built the Shadow Collective, took Mandalore, killed Vizsla with his own weapon and claimed the Darksaber, and fought Sidious himself. He was more dangerous after the injury than before it.

The reason is temperament. Maul was extraordinarily arrogant and utterly confident in his own abilities, and that confidence meant the injuries never touched his use of the dark side. He did not interpret being cut in half as evidence that he was less than he was. He interpreted it as something to avenge.

Vader looked at his own body and saw a monument to his failure. Maul looked at his and saw a debt owed to him.

The takeaway#

Two Sith apprentices, the same enemy, comparable mutilations, opposite outcomes. The mechanic that explains it is not physical at all.

Prosthetics only take away what you decide they take away, with the single exception of powers that need living flesh to leave the body. Vader spent twenty years believing he was a ruin, and so he was. Maul spent the same years believing he was owed a galaxy, and grew into something closer to it.

This is all Legends framing, drawn from the sourcebooks and novels that filled in how the Force actually works. The current canon has never contradicted it, and the character trajectories in The Clone Wars and Rebels line up neatly with it.