Henry, I think the best response would be from the Porsche owners (check their club forums) as a lot of them come with that as an option.
Having said this, and while I do not have personal experience with the 3M though I asked about before - it's all in the quality of the installation, I would think a clear bra cover would be less effective as it has less padding or thickness to absorb any flying debris or rubber pieces from the track when compared to say the OEM type bra. Also, as you get more hits, the clear types will get scratched or torn and thus look very much like pitted paint whereas you just wipe out any dirt that stick on the oem version.
Many of us who track the car use the oem type or simnilar product such as SpeedLingerie. And frankly, as we encounter more R compound tires at the tracks, I would not get on the track without one. I have seen some Porsches use a film to cover the entire front hood as well. HTH