On triple channel single socket boards, very little. The bandwidth is ridiculous enough at any speed. It's likely to matter more with P55, though still not anything to lose sleep over I suspect.
Latency matters with ram, 1600mhz at cl12 is crap but at cl6 is excellent. Latency doesn't actually matter that much either, since again bandwidth is ludicrous anyway. Below 10-10-10 would be a good idea.
For x58 i7, I think the ideal spot is 1600mhz cl8, but this is probably quicker than I'd notice any difference,
If you want the obvious answer, some cost 3 times as much because people will pay three times as much for slightly better numbers.