Scawen has already said that new GTR interiors are on the way. I'd wager that they're already done and are just waiting to be released in Patch W with a bunch of other goodies and non-compatible changes.
As for the XR, XF, UF, FXO, RB4, FZ, etc...they are road cars. Slightly modified, but road cars none the less. In all likelyhood they would have idiot lights, though probably not airbags (not that I want to see those in game, necessarily). If these were really race prepped cars they would have hood pins, window retainers, slicks, stripped interiors, etc. Nobody has a stereo and street tires on their race car, but plenty of people DO take their weekend car to the track and that's what I believe the devs were shooting for when they designed the road cars. The ONLY thing that makes these cars seem like race cars at all is the adjustability of the suspension and gearing which I'd bet money will go away sooner or later.