I have a mid 2009 MacBook Pro and had it upgraded to 8GB RAM within 2 years after I bought it. At an authorized retail store, where I bought it. No problem. Except that it also has the original 160 GB HD. Performance is not great the last year or so, wondering if it would improve with a 512SSD HD, and upgraded OS. Currently 10.6.8. I did replace the battery once, but did not notice extra heat with the upgraded 8GB RAM.
The 8GB RAM has served me well. I'm no techie, but I would guess that the OS, HD and RAM all would affect performance in different ways. Anyone know???