Actually, there are a lot of chemical additives that you can add to rock salt to bring down it's operating temperatures.
Here in Indy, we're using liquid Calcium Chloride as a pre-wetting agent, which drops the effective temperature down to -5 or so.
It's purely related to the number of ions the salt breaks down into, so yes, using a salt other than sodium chloride can have some additional effect. Only a few salts are economically feasible without being significantly worse environmentally than NaCl, so they are usually used for sidewalks where the bulk cost isn't relatively important.