I want to upgrade my mid 2009 unibody macbook pro to 750gb-1tb?

Nobody seems to know the answer to this. Apple cant tell me, geek squad cant tell me. I read that you cant because they are typically 12.5mm and you need 9.5mm size to fit in the mac. But I found a WD scorpion blue 750gb that is 9.5mm, but amazon reviews say it doesnt work on the mid 2009, says its incredibly slow. Why is that? What can I do? Am i stuck with a 500gb or do I have options? P.S. dont want anything Seagate, had a bad experience with their external.