I tried something a little more crude (2 little bottles cut to fit on bottom of my shoes) in the WindTunnel that the Cruzbike Team and myself were at many years ago (A2 WindTunnel in Charlotte, NC). We saw no effect - actually there were detrimental
I also shaped some smaller ones (that just covered the flattest parts of my shoes) out of foam and wrapped them in CF a couple of years ago and tested them on my local track versus same shoes without. I also could not see any difference.
IMHO there is just too much turbulence around the feet with them spinning as they do for shoe fairings to make a difference.