I found this stuff a while ago, but forgot to tell anybody. It is from the US Plastic Corporation, listed on their Flexible Hose and Duct
page. There are a few options that qualify for different heat ranges, and I believe the "RFH General Purpose Flexible Hose" would work nicely for FWI and similar air intake setups. It can range from -60 to 275 degrees Fahrenheit, which seems like it might be able to withstand underhood temps. Maybe I'm totally wrong, though. Perhaps someone can put together a group order, since the minimum length available is a 25 foot section. Hope this provides us with another option for intakes!
EDIT: It is also my understanding that this hose/duct is smooth-walled.
SECOND EDIT: I just checked Intense'* website, and their tubing has a temperature rating of -40 to 250. The tubing listed in my link exceeds these specifications, so if it is smooth-walled, it is only about 1/3 the cost per foot of Intense'* tubing.