Unless you've got 1333MHz with a much lower latency than the 1600MHz Apple's using, it will be slower. But when applications completely use up the RAM on your Mac, the hard drive will be used as RAM - which is about as fast as FPM DRAM, or SDRAM if it's an SSD (FPM<EDO<SDRAM<DDR<DDR2<DDR3).
Obviously it will be slower, but how much? 3% 10%? That's really the question. I purchased it because I got it for $48. And my wife has my 2011 Macbook Pro, so if it is a 10% hit I'll purchase 1600 and move the 1333mhz to my Wife's Macbook Pro (not that she really needs it, but what the heck).
And actually, it is lower latency. Generally the 1333mhz "Mac" RAM has a cas latency of 9, where as 1600mhz is 11. So that's what I am going to find out, what is the true "hit"? Does the slight drop in speed, but lower latency equal a minimal hit or does it really make a huge difference?
EDIT: And you really don't need to "explain" to me how a computer works and what happens when you run out of RAM. My point about the "whether the 16GB makes up for it" is that I regularly see Mt. Lion using 2+ GB of RAM add in the fact that 512MB is being used by the GPU means you really only have 1-1.5GB available for applications and what not. If a benchmark is using up more than 1GB then by just adding more RAM in general (whether 8GB or 16GB) would more than make up for the slight loss in memory bandwidth.
