From what I've read and experienced, FFB Sensitivity 10 is the most sensitive, 1 is the least sensitive.
Generally, people with Thrustmaster wheels use high sensitivity and low torque. If you turn down the sensitivity you are missing subtle details of the track, and the details it does pick up will be slighter, especially if you run a low torque setting like most Thrustmaster users do, because they are particularly strong wheels. I think reliability issues also arise with torque settings above 5 or 6. Just too much load for the motors.
If you set torque low and sensitivity high, you're attempting to pick up more details of the track while keeping torque low enough to not muffle their impact by generating substantial feedback for tiny undulations. If you're simply not interested in all the tiny details of the track, keep sensitivity low, but I don't ever recommend high torque, especially on the T300 RS GT. Not only will it die sooner, but you'll be interpreting small bumps as large bumps, undulations, etc..
I usually keep torque on 5 and sensitivity on 9, but for certain combos I turn the torque down to 4.