Orchtrpt,
There have been recent studies that show that potassium has shown the ability to lower blood pressure. The greatest decreases have been shown to occur in patients who are hypertensive to begin with. With normal blood pressure considered to be 120/80 those who are hypertensive and on K supplements or diets rich in K showed a decrease in BP of about
2-4/1-2 with normal patients showing a decrease of 1-2/0.5-1.
As far as popping a couple of tabs before an audition; I don't think it would cause a normal individual (kidney wise) any problems as potassium is cleared by the kidneys very rapidly so there is no extreme bloodstream buildup. Whether there is any benefit to calm oneself before an audition or performance, who knows? The placebo affect can be a very strong force. As George Constanza once told Jerry Seinfeld: "it's not a lie if you believe it"!
As far as eating excessive amounts of bananas go; bananas, although very high in potassium, it is also very high in sucrose (fruit sugars) and has a very fast effect on raising blood sugar. So if your one of those who gets sugar jittery, it's probably not the way to go.