The “mystery” adjuster on the XT’s that SLX doesn’t have adjusts the free stroke of the levers, that is to say how far the lever travels before the pads come into contact with the disc, and they do make a difference if not a massive one
Various people had a look and said “It stiffens up the lever feel” “It reduces the servowave effect” “It adds extra lever travel before the pistons move at all” and “it retracts the pads further from the rotors for mud clearance” (all the magazines seem to think that’s what it does- mine didn’t)
Anyway- the main thing I found was that they developed a sticky piston which needed a replacement caliper as you can’t buy the little seal seperately… And that when I sold them and bought my Oros I got a more powerful, more reliable, more consistent brake. So all’s well that ends well 😉