I have just purchased an Antec P182 case and in the manual, when talking about the Top and Rear exhaust fans, it says "We recommend that our users set these two fans to Low in order to cool the CPU more effectively". This doesn't make sense to me. Surely higher fan speed = better air flow = better cooling?
Perhaps if they're both running at high speed they cause turbulence. Realistically I still think they should cool better running at medium than low, but I don't know about high vs low - that might be in there to cover up the fact that the top fan is extremely noisy at load.