Strokeside,
Definetly, the difference in *memory access performance*, comparing cas 3 with cas 2 at the same speed (FSB) should be bigger (in fact, around 15%). BUT, a computer is the sum of its components, and rel life or generic benchmarks do not test ONLY memory speed... so in real life and for this scenario, the difference may vary from 0 to 15%, all other system specs equal.
If you are measuring with sisoft's Sandra memory benchmark, and getting only 1% difference, then you are right, your BIOS is probably setting CAS3 even if you say you want CAS2... (check the SPD settings).
Regarding this thread's initial question, I agree in general FSB is more important than CAS setting. By this I mean, it is usually better to drive a locked system to it's FSB limit at the more relaxed memory timings than stop overclocking just because you do not want to concede a bit on the mem timings.. as with everything, there CAN be exceptions.
Regards
FTC