I remember reading about these in my early days of British cars and thinking it would be soooooo verrryyyy cooooool to have one. This was the late 70s early 80s, they were no longer available new, and there was no internet to help find them, or to share information like this about them either. I think one of the classic sports car mags tested one of these recently, maybe on a Sprite, the period ads said somthing like a 45% power increase, as I recall they did not get near that, maybe 20-30% don't remember exactly, but I do remember not as advertised, but nothing to sneeze at.
I have also read the the vanes were basically an oiled paper or cardboard type thing, and didn't last long. I still think it would be cool to have one, neat period accessory, the seemed to be popular for Spridgets and TRs, but they made them for a lot of other imports as well.