Are they good? That depends on the goal. If you're looking to improve the ride, absolutely not. If you're looking for a little more lift, then yes, you'll get about 2".
Procomps suck though. Get a full length add-a-leaf if you get one.
They're designed to curve the stock spring to a slight curve to gain 2". I really doubt they'd do anything but maybe make a 4" pack stiffer.
Sure you can stack more than one. Dunno what you'd gain, but no reason you couldn't do it assuming your center pins and u-bolts are long enough. If you're SUA, it does mean less clearance under the pack though.