Imported Cars of Stamford in CT has an ex-Ferrari tech. He's a good guy, technically and personally, but their alignment machine is a bit outdated. To be honest, I would rather trust offical BMW sources, i.e. your dealer since they have the offical alignment procedures and specs. Just don't be polite, know the facts, and make sure they are not BSing you.
You dropped your car right? I believe that the front cambers will be more negative than BMW suggests, by about 0.1-0.2 degrees, but that's ok, just more wear on the tires. Rears should be within spec even after the drop.
Last word of wisdom: TEST DRIVE BEFORE YOU PAY!
E92 M3 DCT