I would agree with you, anger and all emotions used by the Sith are simply means to an end. This is why I believe there must be other ways to achieve dark side power, perhaps without emotion and instead through meditation, or by feeding off the emotions of others.
Anger is the gateway to hatred which can be directed towars a lot of things, towards the enemy, towards weaker sith, even towards the sith himself/herself it doesn't really matter though, as long as there is enough emotion it is fine. You can compare it to a boxer receiving a pep talk from his coach before a match but in this case emotions are not only a drive but, also the fuel for the dark side which the sith will use.
You are taking the subject of anger as we normal people do, our bodies produce adrenalin we stop getting tired but become ragefull and lash out to, eventualy get exhausted after putting our bodies through an unnessecary ordeal but, it isn't the same with sith it is more of a constant thing, they always feel anger, always feel fear and hatred, these empower them they also corrupt them and deform them in some ways, of course there are exceptions to this but im not going to disscuss them here, what i wanted to say was in essence the anger that we feel as real people is different than what the sith feel. It is more like achieving control over your anger and channelling it to a certain object or person. I guess what im trying to say is it is a means to an end not the end itself.