Hi Tim!
As was stated previously, torsion leafs and bars are similar, yet different. In addition to the aforementioned information, you should also be aware that a leaf pack has inter-leaf friction that affects both rate and durability. Torsion bar suspension travel is generally limited to 45 degrees of rotation, where leaf packs are capable of more (somewhere around 75-90 degrees) rotation travel for a given length. Rate predictability (ultimate- not initial) is far better with the bar (either soild or tubular) due to no inter-leaf friction and wear.
Leaf packs, within the space claim available, are far easier to "tune" to a given suspension by adding or subtracting leaf material. The only method I am aware of to tune a round bar is to make it a telescoping tube and bar arrangement with internal splines to "slide" the inner solid bar into the tube, effectifely stiffening the spring rate. This taks space however, and in a VW style front suspension, is, in my opinion, not very practical.
From a production standpoint, I have seen vehicles with as little as 75#/degree twist going up to 7900#/degree twist (M1A1 tank) so I am not sure what you are actuall asking for with that question.
From a damping standpoint (theory only) the single bar will be more easily tuned to the shocks, becatuse of a lack of internal friction.
I hope I have helped....
Best regards,
Bob Sheaves