The numbers for DDR2 should be a little higher

so, 4-4-4-12 might be the limit already. For other params, they're delays the mem controller needs before it can make the data ready. You can try one by one and little by little to decrease them. There's no general numbers you can use.

Keep in mind, unless you play computer games a lot, those tweaks won't improve the performance of your daily use applications.