I got the info about the pressure being different for pneumatic vs hydraulic from a Senior Engineer at Silverline Plastics, who mfg miles of PVC every year. I think he probably said the same thing you did (it's been years ago) and I didn't get the take on it as well as the way you said it. :) Bottom line is don't use PVC for air lines!
You may be able to answer this. Is Rapid Air piping just Pex-Al-Pex? I know the Rapid Air piping has an aluminum core in it, but never actually seen pex-al-pex at Home Depot or Lowes. I did replumb our older RV at the beach with Pex, and used all brass fittings. Easy stuff for a hack plumber like myself to use....lol