XPS 15 L501x memory upgrade configuration demystified?

Ok guys, have an XPS 15 L501x notebook with no complaints about it til now...

I need to upgrade to have 8GB of RAM and noted the following:

- The laptop comes with 2, 2GB DDR3 PC-10600/1333Mhz DIMMs CAS LATENCY 9 (CL-9) (in my case manufactured by Nanya Technology for Dell).

- The chipset for the L501x is in my case at least an Intel HM57, which runs a Memory Bus @1066Mhz (actually dual channel @533Mhz)

-  Also If you take a look at the real memory timings supported by the Chipset indicates support for CL8 timing....(look at the image below).

So the thing is this configuration comes with a set of DIMMS that of course work with no problem but under a clock setting and frequency/speed different wo what the Chipset supports.

Also if I check the memory upgrade recommendation for XPS 15 from the parts website at dell.com, you will see DDR3 PC3-10600 @1333Mhz (meaning CL9 as well).

So.....Have anyone else noted this before upgrading your memory config? Does a claim applies in this case, in order to get the right DIMMs timing ? And finally should I upgrade using DDR-3 PC3-10600 CL9(most common today) or look for DDR3-PC8500 @1066Mhz CL8 or 7.

Thanks for any comments!