Smaller is better.
(CAS / Frequency (MHz)) × 1000 = X ns
(9 / 1600) x 1000 = 5.63 nanoseconds
(11 / 1600) x 1000 = 6.88 nanoseconds
While RAM speed / timings is oft poo poo'd as having no real difference, is supported only when ya don't look to deeply or compare relative price / performance ratios inappropriately. What this comes from is misinterpretations of RAM reviews whereby the benefit of faster RAM is judged against the relative costs. Example....
New RAM platform comes out and reviewer looks at two sets .... 1600 CAS 9 and 2400 CAS 10. he sees that the increase in average fps is only 0 - 5 % whereas the difference in cost of the two sets is such that one is twice the cost of the other . The conclusion is drawn that the improvement is not worth the increase in cost and this conclusion will be parrotted in forums for years. Is that valid now... and if not why so ?
1. When a new RAM platform comes out , yields on the hi spec stuff is very low and because it is rare, the high end stuff will draw a high premium. Years later this premium disappears.... 2133 CAS 9 right now is oft the same cost or no more then $3-4 more than 1600.
2. The RAM cost is red herring in that it's not just your RAM that goes faster, it's your whole system that sees the benefit so the cost of the build is what matters.
3. Average fps is not where we see the effect of faster RAM, the real impact is in minimum fps.
4. If your GFX card is the limiting speed factor, the impact of faster RAM might not show up till you go to multiple cards.
5. The impact may be significant in one game and 0 on another. Obviously in demanding applications like video editing, CAD, database manipulation and such this claim is no longer made much.
So let's look at this:
$155 Gskill Trident 16 GB 24500 CAS 10
$130 G.Skill Ripjaws X Series 16GB
Cost ratio is 20% and no one would argue that it's 19% faster. However, the 970 SLI system with 4690k came in at $1725 and adding $25 and that's a 1.4 % price increase. Will we ever see a 2% or more increase in what we do on our puters ? Most certainly yes.
http://www.tomshardware.com/reviews/32-gb-ddr3-ram,3790-10.html
Here we see an 11% difference in gaming performance in F1 ... in Metro we see virtually nothing. That represents the two extremes. More typically the differences range from 2 -5 % and while that might certainly be describes and not very significant, the 1.4% difference in system cost is even smaller. And before anyone mentions IGP, the test was done with a 4770k and 290x
Here's some test data where they looked at minimum rather than average frame rates
http://www.anandtech.com/show/2792/12
22.3 % (SLI) increase in minimum frame rates w/ C6 instead of C8 in Far Cry 2
18% (single card) / 5% (SLI) increase in minimum frame rates w/ C6 instead of C8 in Dawn of War
15% (single card) / 5% (SLI) increase in minimum frame rates w/ C6 instead of C8 in World in Conflict
Also see
http://www.bit-tech.net/hardware/memory/2011/01/11/the-best-memory-for-sandy-bridge/1
In short..... when a new platform drops and prices for the high spec stuff is high, it's hard to make a recommendation to get faster RAM for gaming boxes. For tough applications, especially in a production environment, it's not hard at all. However, once a platform has matured as we are today, buying 2133 CAS 9 for $74 over 1600 CAS 9 for $71 is the proverbial "no brainer". As to whether to "go higher", 2400 is a reasonable price premium for a high end gaming box; higher than that it's must tougher to justify.