Here's an answer written by Willie Worthy in Four Wheeler Magazine's Techline.
"You can gain some additional height by adjusting your trucks torsion bars, but remember, any time you get more downward wheel travel by either using a stronger torsion bar or putting more tension on the bar, you're reducing the amount of wheel travel available in the other direction. This makes for not only a stiffer ride but the reduced travel can hinder off-road performance by holding the tire in the air. ?If the tire isn't on the ground, it doesn't grip very well."