Yeah don't even worry about front "downforce" with the stock or aftermarket cosmetic pieces. All that can really be said for those is that the stock sport front-spoiler skirt (and the Nismo setup) are "zero lift", so they at least block off upforce
To get real downforce, you really need a setup like mentioned in the earlier link to Megan's journal, where you build little frame rails that screw under the car where the undertray does and build a huge solid platform. I haven't heard of or seen any ready-made commercial options of that nature for our car (replacing the undertray basically), although they wouldn't be that hard to make, just no real market for it.