I think the best way to improve right hand picking speed is through tremolo excercises, because when you think about it, if you can't play the same note at a certain tempo, how would you be able to play a scale at the same tempo? What I do when I want to take on a new speed is I first work on tremelo picking at that speed, just open strings, then notes, then moving up a scale. When it gets to the point that that speed is a breeze on my hand, I start trying scales or whatever. It comes a lot easier. Does this make sense? If not just ask I'll try and clear it up.