How fast you go down hill is in direct relationship to how well you can negotiate the next turn. Anyone can go really fast downhill but few can get that next turn perfect. I would work on that next turn untill you get it perfect, then gradually increase your speed. BTW just work on that section of the track so you don't wear yourself out on the rest of the track. Jump off and ride back and do it again, and again until you get it. As far as body position, I'd stay standing, bent knees, towards the back of the bike.