I think there is evidence that for the Sith, passionate fiery anger initially leads them to fall to the dark side, but this eventually tends to solidify into more of a cold contempt for others, a kind of detachment based on seeing everyone as inferior, more just irritants and obstacles than people worth displaying overt emotion over, even if behind that lies a simmering hatred for others rather than genuine indifference of the sort one might feel towards inanimate objects. (Sidious and Dooku seemed to behave in this sort of cold contemptuous way most of the time--in The Clone Wars episode "Witches of the Mist", Dooku is training his new apprentice Savage Opress and the contrast between the apprentice's passionate anger and the master's cold manipulative attitude is quite apparent, see the clip here).
And along with detaching from caring about others, there also seemed to be a kind of hyper-focus on power as the only goal worth pursuing (as suggested by 'through power, I gain victory' in the mantra you quote), so that Sith did have the ability to detach from any emotional reaction that didn't in some way service this goal, becoming more calculating and machiavellian, avoiding overt expressions of rage when they weren't useful (as with Sidious constantly wearing the mask of the good-natured Senator/Chancellor Palpatine), only allowing "cold" hatred to turn to "hot" anger when doing so would help fuel some dark side power they wanted to use for some goal (like the Emperor acting more passionate whenever he was shooting force lightning at someone). The novelization of Revenge of the Sith shows Sidious advising Anakin to find a "cool and remote" place inside himself in order to leave behind his old feelings of loyalty to the Jedi and pledge himself to the Sith:
"It's just—it's not ... easy, that's all. I have—I've been a Jedi for so long—"
Sidious offered an appalling smile. "There is a place within you, my boy, a place as briskly clean as ice on a mountaintop, cool and remote. Find that high place, and look down within yourself; breathe that clean, icy air as you regard your guilt and shame. Do not deny them; observe them. Take your horror in your hands and look at it. Examine it as a phenomenon. Smell it. Taste it. Come to know it as only you can, for it is yours, and it is precious."
As the shadow beside him spoke, its words became true. From a remote, frozen distance that was at the same time more extravagantly, hotly intimate than he could have ever dreamed, Anakin handled his emotions. He dissected them. He reassembled them and pulled them apart again. He still felt them—if anything, they burned hotter than before—but they no longer had the power to cloud his mind.
Also note the way that in Return of the Jedi, as soon as Luke really taps into his passionate anger towards his father and chops off his hand, the Emperor seems to expect that he's fallen to the dark side and this will immediately lead him to value power above other things: "Your hate has made you powerful. Now, fulfill your destiny and take your father's place at my side."
I think this transition from passionate anger to more controlled contempt and calculated pursuit of power is also reflected in Yoda's statement in The Phantom Menace:
Fear is the path to the Dark Side. Fear leads to anger, anger leads to hate, hate leads to suffering.
George Lucas elaborates on this a bit in this filmed discussion with the writers of The Clone Wars:
A relevant quote from the above:
"You got the dark side, the light side, one is selfless, one is selfish, and you wanna keep them in balance. What happens when you go to the dark side is it goes out of balance and you get really selfish and you forget about everybody ... because when you get selfish you get stuff, or you want stuff, and when you want stuff and you get stuff then you are afraid somebody is going to take it away from you, whether it's a person or a thing or a particular pleasure or experience. Once you become afraid that somebody's going to take it away from you or you're gonna lose it, then you start to become angry, especially if you're losing it, and that anger leads to hate, and hate leads to suffering. Mostly on the part of the person who's selfish, because you spend all your time being afraid of losing everything you've got instead of actually living. Where joy, by giving to other people you can't think about yourself, and therefore there's no pain. But the pleasure factor of greed and of selfishness is a short-lived experience, therefore you're constantly trying to replenish it, but of course the more you replenish it, the harder it is to, so you have to keep upping the ante. You're actually afraid of the pain of not having the joy. So that is ultimately the core of the whole dark side/light side of the Force. And everything flows from that. Obviously the Sith are always unhappy because they never get enough of anything they want. Mostly, their selfishness centers around power and control. And the struggle is always to be able to let go of all that stuff."
Incidentally, this stuff seems strongly reminiscent of Buddhist ideas about the dangers of attachment to pleasurable things; in this interview Lucas said his daughter was once asked about their religion, "And she said we're Buddhist Methodists. I said, well, I guess that's one way to describe it [laughs]"
