PVC is designed to carry pressurized water -- it can handle an engine pulling in air, and it has no problem handling heat either. UV it's not so fond of -- but unless you're running a grow-op in your engine bay that shouldn't be an issue.
Though if you want better heat insulation, instead of using PVC pipe, get PVC conduit (grey instead of white). It's designed to enclose electrical cables or hot water systems. It's more expensive than regular PVC...I'd just use a ceramic-based paint and regular PVC; the combinatin is less heat conductive than aluminum.
Either PVC is lighter than aluminum pipe, and a lot cheaper.
Hey, Peque+/-o...a 3" Henry Vo won't fit in an automatic 5S-FE? I've been mapping it out -- planning to put a 3" in myself -- and haven't seen any conflicts with the transmission.
Where'd you run into problems?