Setting GstInput.MouseSensitivity to 1 in the beta solved some acceleration issues when in 1st person, but didn't have any effect on the Javelin. For the release version 1st person is fixed and doesn't have acceleration anymore, but the Javelin is the same.
I just realized they completely broke the scaling for GstInput.MouseSensitivity in the release though, so I removed it from the calculation and added a note saying it has to be set to 40 for the calculation to be correct. Using 40 doesn't come with any limitations though, so it should work for everyone.
In the beta it was a pure multiplier, but it doesn't scale in a mathematically sensible way anymore.