Ok, so I tried this with a fresh INI and now I feel like the fresh INI defaults feels very off: horizontal sensitivity is too low.
From here on, replicating the settings that the migration code did for me
, makes it feel right again.
So in conclusion, the default (fresh INI) of 1.0 for both X and Y sensitivity doesn't feel right to me, but the ones that the migration code calculated feels right (X = 4.0, Y = 1.0 in my case).
As I said in my previous post, ever since playing every version of ZDoom and GZDoom since 2002, I have never
had to alter the mouse sensitivity settings. It's stayed at 1.0 the whole time and felt completely natural and playable to me. And I've changed mice many times. My point is, I think whatever the migration code calculated for me (X being 4) is as close as it gets to "pre-mouse-cleanup refactor".