Ballistic, here are my T500RS Settings. Just an FYI - I tried a whole bunch of settings from various users, and I'm not sure I felt too much difference from one to the other, but this is what I got used to and have stuck with, so I 'm not sure if it's the best but it's what I've got.
First go to the Thrustmaster PC Control Panel and do any updates to the firmware/software you'll see the check for update button on the bottom right of the Test Input tab. After that go to the wheel rotation/angle on the same page and set to 900. This is a setting I wish GT5 had for the T500, but since it's only 1080 in GT I rarely use the wheel, it's a shame actually.
Next go to the Gain Settings tab.
Overall Strength of Force - 80%
Constant - 100%
Periodic - 100%
Spring - 0%
Damper - 5%
Auto Center Setting - by the game.
I'm also using version 1.10.
Next go into the sim to set up some other settings. Once in the sim, go to options then check your steering and make sure it's all the way to the left to match the cars lock to lock.
Force Feedback should be 4.0 for strength (too much more makes the wheel shake at high speeds)
Dampening should be at 5.
If you are using the "Real Brake Mod" then set your brake force curve factor to 0. I took the real brake mod one step further and also installed a solid rubber grommet in between my brake pedal and where the pedal maxes out at; that was a great $.49 investment from Ace Hardware and from what I've read makes the brake feel close to a Load Cell setup.
I've also found it natural to map the enter/exit/tow button to my PS button on the center of the wheel.
I hope this helps and good luck!