Over at Freesteel, Julian talks about using a "DiamondAngle" in the interval [0,4] for representing a plane angle instead of the usual radian in [0,2PI]. The argument is that using diangles there is an exact floating-point number for north/south/east/west, and that conversion to/from diangles is faster because it doesn't involve calling trigonometric functions.
I did a test with this, converting 10 million uniformly distributed radian angles to/from unit-vectors, and ditto for diangles. Converting diangles to unit-vectors is ca 25% faster than calling sin() and cos(), while the reverse transform is almost 3x faster than calling atan2. Runtimes in seconds below:
compare radians to dianglescos/sin 1.13atan2 0.78dia2vec 0.83vec2dia 0.27