Personally, the only time I think about air flow is when I am cutting venting holes. I dont think about downforce, I am making sure enough air is getting into the shell to keep the hot bits cool! IMO the main purpose of the rear wing is to protect my shock towers when I crash!
I think worrying about downforce provided by the shell and wing is, to some extent, a moot point. The benefits of high downforce wings and bodyshells are most noticable at top speed which is only attained for maybe two seconds a lap? Not really enough to worry about for me.