There are some bolt kits for the rear, and pad kits for the front to lower the car about 1-1.5". It does not increase the spring rate, AFAIK, though. So technically the car would be more likely to bottom out, etc. But I may be mistaken on this.
There are differing spring rate springs based on suspension options, something like a Z51 or Z07 spring might be firmer. I don't know if there's a big aftermarket selection of springs, as it's a rather unique design.
FWIW, my car with just the rear lowering bolts (done by the owner before me):
