I'm getting a headache thinking about this.... <img src="/forums/images/graemlins/lol.gif" alt="" />
I think the T-bar spring rate is not linear...The difference in force needed to compress the suspension increase as you compress the suspension. When you crank the t-bar, you basically increase the force needed to compress the syspension. That's what people refer to as "preload".
Another way to look at is the resistant of the t-bar from the rest state increases as the bar is twisted is not even. As the t-bar gets cranked, more down force is needed to move the suspension downward. This translates in to stiffness.

In the real world experience, I cranked the t-bar temporary to compensate for the 35" MT/R. I notice imediately that the ride is stiffer. When I go through speed bump, the wheels wants to drop down a lot quicker than the truck. Gas shock increase this effect further ( I have Edelbrock ) This also make a rough ride. When I'm on the frwy, the same thing happen also but at a much higher speed.
Overall, cranking the t-bar is probably ok to a certain extend. I don't recommend people do it to fit 35's tires with 2" body lift.