In some cases, the boosting and shielding work, and can give 6db gain. In other cases, putting any kind of metal near the radio causes dramatic signal loss. Apparently some radio circuits cannot reject their own reflected signals, and so are overwhelmed by the strong reflected signal from the nearby foil.
The best way to test is to start a big file copy over the network, and to adjust the foil during the copy, watching the progress trend. A 6db gain or loss is readily noticeable.
Since no wireless router broadcasts a signal out the bottom, placing a piece of foil under the router will always be a win if your router is not talking to radios on lower floors.