Ok, so if you just do this:
[Controls]
fIronSightsPitchSpeedRatio=1
fPitchSpeedRatio=1
You get the same pitch as yaw with the same value. As we can set a separate Y sensitivity this isn't really needed though as it is an extra step for most people.
If you also do this:
fMouseHeadingXScale = 0.021
fMouseHeadingYScale = XX
Where XX replace with:
.03738 for 16:9 resolutions
.042 for 21:9 resolutions
.028 for 4:3 resolutions
.0336 for 16:10 resolutions
You will get a pitch sensitivity which is faster by the aspect ratio. So for instance 0.3738 will give you a pitch 178% of the yaw.
If you just do the fMouseHeadingXScale thing you will get a pitch that is 56.25% of the relation between the numbers. So for instance 0.042 will give you a pitch which is 1/(0.021/0.042/0.5625) = 112.5% of the yaw.
Neither of these cases make any sense. Setting the ratio to 1 does however, but it's just as easy to set a different pitch sens.