Mobile video cards are generally less powerful than their desktop counterparts. Therefore, the Mobility HD 5730 would be less powerful than the desktop version of the HD 5730 (if it exists).
If I had to guess the Mobility HD 5730 would be a little more powerful than the HD 5670. See following benchmarks:http://www.xbitlabs.com/articles/video/display/starcraft2-wings-of-liberty_6.html#sect1
I would guess maybe 10% - 15% better performance at most. You should also realize those benchmarks are based on a PC with an Intel Core i7-975 Extreme Edition processor.