I disagree ... I DON'T believe heat can have a tangible effect on HD performance.
Hard drives are mechanical ... they spin at a certain speed, their voice coils move their arms at a given speed (seek rate), etc.
If the temperature got hot enough to throw this precise mechanism off enough for you to feel a performance difference, then your system would hang, crash, blue screen, ie.) experience the effects of a hard drive failure ... at least until the drive cooled down some.
Heat can shorten a HD life though. Heat affects magnetic charges and I would surmise that too much heat could damage the platters (magnetically) causing bad sectors or even the hard drive to go kaput. Also, I can't imagine that extreme heat is good for the bearings/motor mechanism either.
My general rule is if the HD is hot to the touch then it's good to do something for the integrity of the drive and data. If your HD is only "warm" in your case then I would think it would be fine.
BTW - My one system has 3 hard drives in a bad case and they get so hot I leave the side open. My other system has 3 hard drives in a nice Antec case that has a fan that keeps them barely warm to the touch.