Don't know about the downforce stuff but in my view they just give a little extra protection and it may be enough to glance the odd stray bit of rock or metal from damaging the sump causing you to drop your oil on the motorway at 70 mph. I guess that it deflects some water away too, although I would have thought that is a lesser issue
In my view, even at standard prices (a tanks worth of fuel for both side pieces and the centre section, or less than half a tanks worth for the centre section only) it's worth it.