great song to practice fast downstroking is like light to the flies by trivium. its a good warm up song too

same here. i used to play low too, like 5 years ago when i 1st venture into electrics. it helped a lot with fast downstroke song. but after a while, i realized it's not really efficient when you start soloing and playing a lot of alternate picking with palm mute(like metallica's battery). so from hetfield's low(wrist around hip), i switch to hammet's low (wrist around waist) which i think is a more controllable position for me. in this position, it's hard for me to downstroke fast at first, but after a while, i get used to it. but i still can't play masters or battery sitting down  .