Expertise needs to be adjusted thats all. It should not INCREASE your damage, that is what your primary stats and secondary stats are for ! Expertise should only lower dmg done to you, crits chance etc..
This is why the gap becomes so BIG between newly 50's and BM geared players, not only do they have the best mods with your primary and secondary stats, they also have a ton more expertise then you which further increases there damage WHILST lowering alot of dmg done to them.