Ok, I have pics, but need to crop & post them. I have taken a few measurements and done some calculations that I will post before I forget.
Using a kill-a-watt, the system uses about:
65 watts at idle
90 playing videos
130 watts in 3D games
The fan is about 18 mm from the duct. If we use an idealized system, we should be able to figure out how much air that passes through the CPU cooler would be forced out of the case. The whole area that air is forced into can be thought of as a cylinder (AKA 'can of soup' shape). See wikipedia picture.
The bottom of the cylinder is the fan pushing air in. The top of the cylinder is the mesh-covered 120mm hole in the back of the case. The 'sides' of the cylinder are the open area between the fan and the duct, where air can escape back into the case. By bringing the duct and the CPU fan closer, we reduce the open 'sides' of the cylinder ('h' in the picture), and we should be able to decrease the amount of air that escapes into the case. This means more air would be ejected out of the back of the case.
If air escapes in any direction it can from the HSF, the current system should eject about 53% of the air that is pulls through the CPU cooler. If the empty 120x20mm fan frame (AKA duct) was replaced with an empty 120x35mm fan frame the ejection fraction would increase to 87%
. The concern is that objects close to a spinning fan can increase noise
. I will only make the switch if summer temps require.
through-duct escape area / total escape area
(120mm circle * mesh resistance) / ((120mm circle * mesh resistance) + (cylinder-sides area))