Steering Sensitivity:
This affects the response time of your steering - basically how fast you turn the wheel. At very low settings the car becomes extremely sluggish and difficult to control. At very high settings, the car becomes ultra responsive and eager to corner. Too high and the car becomes 'twitchy'. At high speeds with steering speed factor set to high, however, you are able to keep control fairly well and get very responsive controls. Setting this to high makes driver error less forgiving, however. At medium you get excellent stability at the loss of some corner-in at high speed. Medium is far more forgiving to driver error. I reccomend medium to high setting for this, depending on your cornering tendency and personal skill (high is harder to control).
Steering Linearity:
This is hard to describe - it affects how profoundly your car turns. At minimum, the car has a more gradual turn, giving a more 'Forza' feel. At max, your turn is much more prounounced, giving it more of a 'NFS' flavor.
After some further testing of this setting on Sebring (so cool that this track is ingame!) I'd say that this setting is more "analog" at low settings and "digital" at high settings. Setting linearity to low will allow for more control with the analog stick (like Forza). Mashing it down all the time actually caused me stability problems, but it also allowed me to baby the analog stick for improved stability. With linearity set to high you get that NFS feel for sure, it's very easy to control. This comes down to flavor. Lower this if you want to use more of the analog stick and have more control, raise it if you mash the stick to the left and right and don't worry about finesse and tiny adjustments. Right in the middle feels good for me.
Steering Speed Factor:
This one fixes a lot of problems people have with the game. At low settings, the car very easily fishtails at high speed and is almost impossible to control. At highest settings, it is much easier to maintain control of the car at high speed and recover. The higher the setting, the less twitch you get at high speed. I reccomend maxing this one all the way out.
Why these settings work:
These settings work well for me - I get stability at high speed and great in-corner performance. I can drift or hard corner as needed and if the tail waggles at high speed it's much easier to bring it back in. I hope this helps, and if anyone figures out something better please post! Happy driving out there!