The question is not so much whether foils can work .. but by what mechanism.
One should note that, in some cases at least, the foil is there either because it looks nice (ie marketing ... a bit like most of the spoilers one sees on the boot lid of many production sedans ...) or the designer couldn't do the sensible thing and stretch the wing.
An interesting topic which, one hopes, will be thrashed to death in this thread ... perhaps some of the site's other practising aerodynamicists might like to wade into the fray ?