How accurate is System Report on RAM speed?
This depends mostly on the capabilities of the FSB and the RAM itself.  If, for example, you install a 1600 MHz stick next to a 1333 MHz chip, most FSBs will time at 1333 for both, if it is even capable of doing so.  Most FSBs also have a maximum clock speed as you indicated in the comments. 

The FSB is not likely fudging the numbers.  There is a chance the 'limit' imposed by KB HT1270 is a soft one.  Though my 2008 MBP had a limit of 2x2GB I ran it for years with 2x4GB sticks.  I couldn't say for certain regarding your specific model, but I'm inclined to disregard this list.  

Don't go crazy though, there are some hard limits on the size that will be accommodated, which you've likely reached with your 16GB.

Additionally, in my experience, transitioning from 1333 to 1600 typically doesn't produce noticeable speed increases, though the doubling of the capacity would.  If you're noticing this under moderate load with no paging, it's likely the effects are psychosomatic. :)