the only true thing i can attest to the "hump" spoiler is it will avoid excessive dust build-up on your rear glass as i have mentioned in your other post.. i know this for a fact as i have compared this with my in-laws whose 4runner doesn't have the hump spoiler.. just looking at the picture will tell you that it will direct air to your rear.. as far as "downforce theory", here's a good read...
Spoilers, what are they good for. - Car Forum Archives | Car Forums...
Spoiler aerodymics rear spoiler design and construction