Fully inflated those bags raise the rear of my car an inch or two by taking some of the load off the springs. The ride quality is horrible with them set like that so I wouldn't recommend them for what you're looking to do. They're great for drag racing though.
Do a forum search for lowering springs, there'* a link to a company that makes custom springs in one of the threads. You just tell them what car it'* for, the amount of lift you want over the stock springs, and the spring rate you want (FE2, F41, a certain percentage increase over those...). I got a quote from them a while back, it was pretty reasonable, but I can't remember what the exact website was.
1996 SSEi: swapped INTENSE 4t65 trans, INTENSE GEN3 LSD, 3.69s, 3000 stall ATI converter, S1X cam, WBS stage 3 intercooler, full P&P gen3 M90, 2.8 MPS, N* TB and LQ4 maf, 4" FWI, TOGs, full 3" exhaust, and a DHP Powrtuner to keep it all in check