All DDR-2-667 MHz SODIMMs currently are CL5, that is standard.
DDR2-533 is CL4
DDR2-400 Can be CL4 or CL3
Maybe the industry will come out with faster components to allow lower CAS Latency performance in the future. It;s a little up in the air to what extent Apple motherboards will be able to exploit lower CAS latency: there is no BIOS of course to manually set the memory timings, so we would have to rely on Apple building in the capability to automatically detect and reconfigure the motherboard timings.
IN any case, it is unlikely to be worth spending mad amounts of money on advanced RAM, and I would certainly look with skepticism on a small seller claiming performance beyound industry standards... given that all modules have to use chips from the same 4 or 5 foundries...
As usual: choose your Mac RAM from a reputable dealer who tests and guarantees compatibility with your model Mac, offers a lifetime warranty and a no-cost return if it does not work. This would generally exclude eBay, overseas and discount store purchases.