I agree with the OP. Windu's amp in the fight with Palpatine was a one-time only thing, and very circumstantial to say the least. This is very evident in numerous parts of the novelization, like here:
"Before Obi-Wan had left Coruscant, Mace Windu had told him of facing Grievous in single combat atop a mag-lev train during the general's daring raid to capture Palpatine. Mace had told him how the computers slaved to Grievous's brain had apparently analyzed even Mace's unconventionally lethal Vaapad and had been able to respond in kind after a single exchange."He must have been trained by Count Dooku," Mace had said, "so you can expect Makashi as well; given the number of Jedi he has fought and slain, you must expect that he can attack in any style, or all of them. In fact, Obi-Wan, I believe that of all living Jedi, you have the best chance to defeat him." This pronouncement had startled Obi-Wan, and he had protested. After all, the only form in which he was truly even proficient was Soresu, which was the most common lightsaber form in the Jedi Order. Founded upon the basic deflection principles all Padawans were taught—to enable them to protect themselves from blaster bolts—Soresu was very simple, and so restrained and defense-oriented that it was very nearly downright passive. "But surely, Master Windu," Obi-Wan had said, "you, with the power of Vaapad—or Yoda's mastery of Ataro—" Mace Windu had almost smiled. "I created Vaapad to answer my weakness: it channels my own darkness into a weapon of the light. Master Yoda's Ataro is also an answer to weakness: the limitations of reach and mobility imposed by his stature and his age. But for you? What weakness does Soresu answer?" Blinking, Obi-Wan had been forced to admit he'd never actually thought of it that way."
Vaapad's a style that uses the Dark Side of the force (well, more like channeling it) without being dominated by it. More or less like Naruto's senjutsu: he can't let the energy dominate him and be transformed into a frog statue, but when using it right he has an amp that depends on how much energy he gathered.
"Because Mace, too, has an attachment. Mace has a secret love. Mace Windu loves the Republic. Many of his students quote him to students of their own: "Jedi do not fight for peace. That's only a slogan, and is as misleading as slogans always are. Jedi fight for civilization, because only civilization creates peace." For Mace Windu, for all his life, for all the lives of a thousand years of Jedi before him, true civilization has had only one true name: the Republic. He has given his life in the service of his love. He has taken lives in its service, and lost the lives of innocents. He has seen beings that he cares for maimed, and killed, and sometimes worse: sometimes so broken by the horror of the struggle that their only answer was to commit horrors greater still. And because of that love now, here, in this instant, Anakin Skywalker has nine words for him that shred his heart, burn its pieces, and feed him its smoking ashes. Palpatine is Sidious. The Chancellor is the Sith Lord. He doesn't even hear the words, not really; their true meaning is too large for his mind gather in all at once. They mean that all he's done, and all that has been done to him— That all the Order has accomplished, all it has suffered— All the Galaxy itself hasgone through, all the years of suffering and slaughter, the death of entire planets— Has all been for nothing. Because it was all done to save the Republic. Which was already gone. Which had already fallen. The corpse of which had been defended only by a Jedi Order that was now under the command of a Dark Lord of the Sith. Mace Windu's entire existence has become crystal so shot-through with flaws that the hammer of those nine words has crushed him to sand."
Here we can clearly see that Mace was really angry with the "great news" that Palpatine is the Lord Sith, it's not a simple hate, he was absolutely devastated by that knowledge.
"Mace was deep in it now: submerged in Vaapad, swallowed by it, he no longer truly existed as an independent being. Vaapad is a channel for darkness, and that darkness flowed both ways. He accepted the furious speed of the Sith Lord, drew the shadow's rage and power into his inmost center—And let it fountain out again. He reflected the fury upon its source as a lightsaber redirects a blaster bolt."
He was submerged in Vaapad, and combining that with the insane amount of anger he was felling, there's no wonder why he proved a match for Palpatine.
"Palpatine still made no move to defend himself from Skywalker; instead he ramped up the lightning bursting from his hands, bending the fountain of Mace's blade back toward the Korun Master's face. Mace's blade bent so close to his face that he was choking on ozone. "Anakin, he's too strong for me—" This was beyond Vaapad; he had no strength left to fight against his own blade."
And even with this circumstantial amp, he was going to be killed by Palpatine's lightning, he didn't have the strenght to fight the Lord's lightning back.
