The best way to build any progress on any technique is to slow it down to the point where you can spot unwanted tension or wasted motion that will stop you from advancing on that technique. So I'd go way slower than 170 bpm, or even no tempo at all to start out with to make it a 100% possibility that I'm playing the riff clean. Watch your body for tension and always play with the tip of the pick, cutting in any deeper than the tip will slow you down and you'll never get fast. Remember that the best way to hold the pick is whatever way you hold the pick that allows you to do a tremelo and you should hold the pick like that all the time. Make sure you're not moving your right hand too high up between notes/chords when you're downpicking either... this is something that has to be stressed because it's so difficult to develop, so many people do it and I always catch myself doing it when playing along with tunes and I get really into it.
There are three riffs that I use when practicing my downpicking.
That Master Of Puppets downpicked intro riff everyone knows about is probably the greatest one and I have all sorts of combos and versions of it to make it harder on myself and practice all kinds of crap.
I also like to practice with the 2:38 - 2:52 riff in Suffocation's "Synthetically Revived" (the one right after Frank sings "to be reborn!") That one is great because it really puts your ability to switch from tremelo picking and downpicking to the test and is a total mindfuck for the right hand.
The craziest riff that I use to practice downpicking would be a riff from Berzerker's "Massacre" that's like almost 300 bpm (I think it's 280) and all downpicking.
Go here and scroll down almost halfway until you see a line say "must be downpicked", it's the riff right below it.
Basically, always play relaxed, with the tip of the pick, hold the pick one way only all the time (not one way when you palm mute and another when you do a scale), never flex your thumb joint, don't wave that right hand around at all (only what's needed), don't dig the right hand into the face of the guitar, constantly analyze your body and technique, practice real slow and you'll get to wherever you were alot faster and alot better than before. Pretty much goes with everything.
