i did a few lifts on those trucks a few years ago. anything over 3" will just space your ifs down so you can put bigger tires on it. it doesnt add wheel travel or better suspension performance. if the kit adds nice shocks then sure, it will change the peformance. some of them use better ways of dropping the torsion bars, or not dropping them at all. sadly, unless you spend big bucks on a highend long travel kit, or a solid axle swap, you're not going to get much in the better performance for cheap. what you can do on the cheap is try torsion bar keys, a body lift, or bushwacker cut out flares. or combine all these things and use a better quality shock/damper. the rear is easy enough to do blocks, add a leaf, or leaf packs, shackle flip too maybe. im not a gm guy and i never cared much for the suspension on these trucks, or the aftermarket that follows them.
look at superlift,skyjacker, fabtech, rcd, for ifs kits. most any shop in this city can get these.
off road design deals in gm stuff, and off road unlimited does solid axle swap kits.
good luck