when i tightened the torsion bars up on my nissan hardbody, it lowered it a good 2~ inches, but made it the most bumpy ride, i had them tightened all the way too. and for the bumpiness, i guess you could get new lowering leafs for the back but i dont know, i just put blocks on my truck.

EDIT: Don't expect your truck to be slammed just from tightening torsion bars and block/leafs.
this is how low my nissan got from just tightening the torsion bars.