Now to get to the point: Why is the GTX 275 that slow? Is it just because it only supports CUDA 1.3? Any tips on how I might get some more performance out of that old piece of crap?
Pretty much. Newer implementations of CUDA have various improvements to the architecture underlying it, which gives you different/better optimizations. That's why there's 4 different kernels packaged into cudaminer (legacy, Fermi, Kepler and Titan).