Each car is categorized to one of the classes of A, B, or C. For example, Honda NSX is a class A, where as Integra is a class B, and Civic is a class C. See the lower left corner of the machine selection menu.
Each time you win in all three classes, extra track shows up, namely, Autumn Ring, Deep Forest, Special Stage R5, and Grand Valley. (This is possible in Easy level.)
Each time you win in all three classes for the extra track, extra car manufacturer shows up in Maker Selection menu, namely, Aston Martin, Chevrolet, Chrysler, and TVR.
When you win in all tracks with Normal level, the staff credit movie starts. Also this movie becomes available from Goodies menu.
"GT Hi-Fi" becomes available from Goodies menu. This is basically same with Time Trial, except the different graphics quality. Track selection is limited to three tracks (Clubman Stage R5, Special Stage R5, Special Stage R11.) The course layouts are same with normal mode, except with different graphics. I'm not sure exactly what the differences are, but I think the resolution of polygon texture is higher (this is noticeable by watching the blue-white chequers of starting line, or the tunnel walls). Also the screen refresh rate is seemingly 60fps. Report says the screen resolution is same with the normal mode (512 x 384).
Downside is that, apparently to provide higher quality image, there are reduced number of objects in landscape.