Sl and stock springs are cheaper from kyb via butpartsby. They are good quality and give the same results as vx which are expensive.
Mv6 you really want face lift lowered sports chassis, but that means doing the front as well, including shocks thinking about it, and then your into set up as well to reset camber and toe to the new ride hight. So might be more than you want to spend. Results are worth it though. Sadly new mv6 lsc are only available from vx, trade club helps, as will Andyc if your not a member I guess.
Cheapest and simplest with slightly firmer ride would be kyb's equivalent be standard (non sl ) springs. And you'll get away with no set up....and you won't have to mess with the ride hight sensor, it should be correct.
(Not sure if pre face lift mv6 springs would be of use? I don't know the pre face mv6 hight and weather the springs are standard anyway..? Think they are...anyone? )
Vx is not a cheap place to buy springs, but if you must trade club or andyc help.