I had a shuttle with a 92mm high rpm output fan (note the system fan is also the cpu fan on a shuttle), it was noisy (actually very similar to a hair dryer noisy), ran 5500 rpm and only cooled 53C under full load.
I put on low rpm high output 120mm fan and adapter. it was much quieter, only turning 1800 rpm and moved LOT more air and cooled 44C under full load.
I guess, it would really depend on the fan and the location as to how well it does. but just from my experiance, the 120 will move just as much air if not more and be much quieter than a 80