Broadwell will likely just be a minor increase in CPU performance; probably another 6% assuming the same clockspeed since they have not said a word about improved CPU performance (just like Haswell).
The main goal is further reduced power consumption so that they can be used in tablets. The biggest improvement will be the graphics core which will probably be another 30% or so. If you are looking for a big boost in CPU performance, then you may need to wait for Skylake mobile CPUs to be released in 2015.