So OK, if that's exactly what boost does, if I enter say a number like 9999 into boost, and the maximum stick deflection value is lower than that, then shouldn't the number default to the highest? (Since the ST should know this value?)
The point being to work back from that number.
If the number instead represents a percentage, then shouldn't the maximum be 100 and not 5000.. Or 9999
It will automatically default to maximum stick deflection, if your value is too high. However, there's really no reason to use such a high boost. Let's say you were to find the magic number, and get 90% deflection immediately from boost.
Even with the lowest sensitivity, your aim will jitter out of control from the slightest touch. The problem is not the XIM or your mouse. It's that the stick resolution is too coarse to make that a legitimate method. The absolute smallest movement possible on the stick would still cause a massive skip in your aim.
On PS4 it would be even worse, as it has HALF the stick resolution of the X1. Think of it like screen resolution. The bigger, the more pixels you have. Lower = less pixels. If you could only move your mouse one pixel at a time, which would be more accurate for fine movements? More pixels. Higher resolution.