I wouldn't be that sure about this. With C7 you have no way of air escaping the route where on one side it has an inlet mesh and on the other it has radiator with regular surface.I'm assuming due to the height of the rear i/o the 120 fan will be that close to the grill it will make a racket.
When you put 120mm there, then with those IO connectors underneath there's still a lot of area around the socket that air can escape so you're not forcing the air against the IO connectors.
Also the difference is in RPMs of the fan - 120mm fan will be something like 1300RPM while 92mm fans may spike up to 2700 RPM for example. The 120mm fan with half the speed of 92's will not be starved and at the same time it will use it's bigger area to pull the same amount of air.