Hi Giovanni. I’m not familiar with Brad Davis’ routine described but I can speak to some of what you describe from the downpicking camp.
Muting strings may help with focus and analysis of one’s right hand technique, probably doesn’t go much further than that however in terms of developing speed.
You mention slow bpm practice and muscle memory being ingrained - that is noteworthy and worth some discussion.
There is merit to slow bpm practice and analysis when working on development of right hand technique but this also has some hidden traps you have to be conscious of.
How many people fall into the trap of practicing lines as straight 8ths slowly, develop corresponding picking motions, then fail miserably as the metronome ramps up - MoP is the prime example but the concept applies broadly.
Take MoP for example, you learn this as straight 8ths slowly, there is a picking motion associated with that approach, then you try to apply that picking motion at speed and it doesn’t work. How many fail at this and blame their downpicking? Downpicking is generally not the issue. It’s not a straight 8ths riff to begin with - your learning the wrong picking motion slowly and then trying to apply that picking motion at speed - which is the reason for failure.
James is emulating a kind of a drum drag/ruff type of rudiment for this. The 2 notes on the E are like a couple quick grace notes and then you hit the A with a bit of accent. The riff actually has a groove to it that has to be emulated to get it right and get it at speed instead of mindless straight 8ths that everyone is so inclined to practice. Yes - there is just as much a difference in the downpicking mechanic of straight 8ths vs. emulation of a drum drag rudiment as there is to a drummer doing the same with their sticks. 2 different things altogether but both can be characterized as 3-note groups. 2 very different ways of achieving 3 notes however and certainly yes, it’s all downpicked; no, there are no sweeps.
Learn it slowly with the correct drum drag/ruff type of picking mechanic and you will get it accurate and at speed but learn it slowly with the wrong straight 8ths mechanic and it won’t work at speed - this is what seems to baffle many.
But rotjab, when I slow the song way down, it sounds like straight 8ths - yes it does. The reason is when James plays this at a very fast tempo and the timing between notes is minimized - it morphs into something pretty close to such. But practicing slow as a set of mindless, grooveless, 3 note 8ths - 1&2, &3&, 4&1, &2&, 3& 4&…is an absolute recipe for failure to get up to speed.
Kind of hard to describe but easy to demonstrate on video, I’m not video savvy so I won’t be making one but hopefully this makes sense and hopefully it helps.
Again, there is merit to slow, deliberate right hand practice but unto itself - but it doesn’t always capture the picking mechanics applied and those mechanics are not always obvious from that type of practice.