Why do you have a need to cool the computer more than the operating system deems necessary for safe operation? Are you overthinking things, or is there an actual reason why you need things to be cooler?
Macs (and all computers honestly) are designed with fans that operate when they need to. Even, let's say, if you add more internal componants/cards to a computer, the fans will still run when the systems gets too hot. It would just get to that point more quickly than in a 'bare' computer.
Unless you are physically doing something to the componants or the temperature sensors to a point where they will not accurately be reading any longer, there really isn't a need to mess with fan speeds.