Here's the dyno graph on the 123d engine
http://www.autoblog.com/2007/03/24/f...-turbo-diesel/
According to the german specs, it looks like the 123d will be lighter than the 135i.
135i:
Leergewicht nach EU1 kg 1560
123d:
Leergewicht nach EU1 kg 1495
So about 145 lbs lighter.