I asked the same question in the past and also did not get an answer. Here's the best I was able to come up with.
According to the Tire Rack website, BBS LM wheels are 25.5 lbs front and 29 lbs rear in stock, 18" Viper sizes. These are different wheels than the BBS's offered on the ACR, but again that was as close as I could get.
If all you care about is light, I also spoke with John Purner at Complete Custom Wheels. His wheels are available in a variety of diameters and widths. He also offers lightweight "race" wheels and slightly heavier "street" wheels. His recommendation for mostly street driven vehicles is an 18" "street" wheel 1/2" wider than stock. These are 23ish pounds for the fronts and 26ish pounds for the rears (John gave me approximations).
Bottom line, John's wheels are the lightest available.
