At idle the engine does slow down a few hundred rpm with full coarse, and there is an effect on the glide as a result. But of course there might not be enough oil pressure to enable the csu to work if the engine is failed. I am not game to shut the engine off to find out!
I also noticed that the glide was better than the POH indicated it would be: the POH gives a glide angle of around 6 degrees but I did not think it was that rapid in fact, more like 4 degrees.
I was hoping someone out there would have done all this before.