>>>How important is Expertise for PVP?
I find them good enough to keep a full PvP set. Then PvP sessions are always quite random and depending on the opponents you will have, I think the stats would be either usefull or useless.
>>>Ans what a about raid gear into my lvl 40 PVP gear, will i loose out then?
I think it is not worth getting your PvE set if you plan to transfer the mods to your PvP gear. It would be better going to the ah and purchasing the parts: faster and less anoying really.