I think you're on the right track by first playing with a metronome. You can practice with a distorted tone just keep any effects off - no chorus and no delay. Keep it sounding ugly - that's the best way to explain that. You want to be clean and precise without any effects on. Also if you can practice in front of a mirror it will show you if your right hand is following through with consecutive downstrokes and if there is any extra tension in your right hand. Find your low threshold on the metronome - where it's the easiest to play and then keep turning up the metronome a couple clicks at a time until it becomes uncomfortable mentally and physically to play. That is your high threshold. Then you can gauge your progress in regards to speed (you can use that with developing any technique).
I have a free eBook at my website if you care to check it out at:
http://www.guitarlessonsbuffalony.com

Keep at it and all the best,
Joe Pinnavaia
http://www.digital-nations.com/artists/testtuberhino