Wow you are right. That's pretty funny. Tested 360 with a script with 1.0 zoom_sensitivity_ratio and it was spot on. Tried 0.01 and it wouldn't even move. At least it affects all fov equally.
You will have 10 more HFOV when going from 4:3 to 16:10. So, your sensitivity will feel slower ( at smaller degrees of movement ) if you keep it at 2.5. But a 360 turn will take the exact amount of distance on your pad.