I assume you're talking about lowering blocks? That go between the housing and spring?
They're used pretty frequently, especially on trucks. They also get used on trucks as a lift kit because the axles are below the springs in a lot of cases. On our cars they're lowering blocks because the axle is above the spring. Typically you want to make sure that the block has a "button" like the spring does to locate the rear end housing, the better ones usually do. Aluminum isn't a big deal. You of course want to make sure that the u-bolts are long enough, and if you really get crazy with the blocks you may want to use larger diameter U-bolts.
As far as racing, the ideal situation is that you would have your springs arched to the ride height you wanted. The blocks add an additional moment arm between the axle and spring. Not a big deal if they're small, it can be an issue when they get bigger. But yeah, trucks use them all the time, and usually in much larger increments (4"+) for lift kits. But, they also typically use larger diameter u-bolts for the larger blocks to counter the increasing side loads.
1.5" is a pretty big drop for a car, what leaf springs are you running?
