MacBook Pro - Install slower memory than specified?
What will happen?  Either one of the following:


It will work but the memory will run at a slower speed
Your MacBook Pro won't boot


I personally don't use slower memory than what is recommended, but I have upgraded machines that had slower memory and those were working just fine when I did.  However, it's a pretty good bet that if Corsair says it's compatible, it will work.  

If the memory isn't compatible, you aren't going to harm anything.  Your computer won't boot and will more than likely give you a series of tones indicating your memory is bad.