In theory it sounds like the ideal set-up...this too was my lesson learned conclusion, as I mentioned in your other thread , I had a 60 to 92mm adapter. I was in the frame of mind that bigger had to be better (less noise and more cfm), which is the case, but not an efficient plan for this type of configuration, I had more airflow, but much less consentration of that airflow over the core part of the HS, which unfortionately for me resulted in outragious and unacceptable temps...so I think it will have a similar effect for you (80mm to 120mm), in this case you will benefit more efficiency w/ an 80mm fan w/ an adj. rheostat so you can regulate the noise level to suit your needs...and it would probably be around a 5 to 10 degree celcius difference (increase) in temps if you went w/ a 120mm fan, as it was with me (60 to 92mm).
In general, a bigger fan has more airflow; in some cases, A LOT more airflow. But as fan size increases, the size of the motor in the middle increases as well (obviously). And as kdo2milger said, the lack of airflow to the core part of the heatsink results in an "outragous" increase in temperature. So basically, the increased airflow from a larger/more powerful fan is pretty much totally canceled out by the complete lack of airflow to the center of the heatsink. About the 92mm fans though - I'm not sure how the increase in a fan's overall size is related to the increase in the size of the fan's motor. My guess is that the relationship is a) NOT linear and b) different for each manufacturer, etc.
So, in short, yes, an 80mm fan is the best choice for an 80mm heatsink.