Unreal Engine 3 is completely unrelated to Source and their yaw based sensitivity. It uses a fixed base number divided by the sensitivity, and in this case sensitivity is adjusted for the 72 FOV compared to 90 which is default for the engine. The negative acceleration is practically impossible to avoid by hand, but with scripts that move really slow it's possible. I got about the equivalent of the yaw sensitivity you mentioned in your first post when I did the first tests, but the numbers didn't make sense in an UE3 game. So I slowed everything down and got the expected numbers.
If you have a Logitech G mouse or keyboard I can give you a script to verify