Mouse Sensitivity: This setting is very important, and depending on how far right you move the slider, the greater the responsiveness of your character's view to your mouse movements. Note that if you're experiencing a lot of mouse "lag", it is usually due to low framerates - especially when in heavy combat or smoky scenes, or large complex outdoor scenes. Raising the mouse sensitivity value can't fix that - you'll have to reduce/alter other settings to increase your average framerate.
r_multigpu [0,1] - This setting is the same as the 'Optimize for SLI' option in the in-game settings. As such, you should set it there, however check to make sure it is set appropriately for your system (i.e. 0 for single graphics card users, 1 for SLI users). Also make sure to check the r_gpysync value (See below) for optimal performance. Some single graphics card users note a performance improvement when setting this option to 1, however this may induce keyboard/mouse lag. There is further discussion of these settings in the Conclusion section of the guide.
r_gpuSync [0,1,2,3] - This setting determines the synchronization of the GPU (graphics processing unit) and CPU. The values for the various modes are 0 for Off, 1 for Beginning, 2 for beforePresent, and 3 for Adaptive. Importantly, this setting currently defaults to 3 for all users which is a conscious decision by Infinity Ward to balance performance and responsiveness. This is because if set to 0, you may gain performance however your control input (keyboard and/or mouse input) may become laggy as the CPU and GPU go slightly out of sync in places (See Conclusion section). If you are desparate for extra FPS, set this to 0, however if you notice mouse/keyboard lag even at 30+ FPS, then reset to the default value 3 (Adaptive) which keeps the CPU and GPU in sync.
Update: Earl Hammon, Jr from Infinity Ward has written to me to explain the r_gpusync issue. Basically the game's developers are aware of the additional FPS possible when r_gpusync is disabled (set to 0), however because this setting controls the synchronization of the CPU and GPU (your graphics card(s)), disabling this can also introduce input (mouse/keyboard) lag. This is because your input can at times be processed by the CPU faster than it can be drawn on screen. So on balance they chose the default of r_gpusync = 3 to maintain synchronization while still giving good performance. The same situation (input lag) can occur if the 'Optimize for SLI' or r_multigpu setting is enabled.
I hope to be useful..