No, you are missing what I'm saying...
I'm not talking about a case duct (that is to say, a snorkel of sorts leading from the CPU cooler to the rear case fan, for example) all I'm saying is close up the open ends of the heat exchanger (the fins) and make the fan direct
all the air
through the fins of the exchanger. As it is now, air leaks out the gap between the fan and the exchanger, and, further, the open ends of the exchanger fins. If it was closed (ducted,) 100% of the air moved by the 'push' fan would go through the exchanger. Even better would be a push/pull setup (like I have on my 212+, not like Noctua's split exchanger, although it would work, too...)
The way it is now, it would be like a water cooling loop with a bypass... only allowing say 60% of the water to pass through the radiator, the rest just bypassing the radiator and going back down the pipe to the CPU again.... not very efficient.
I have a dream... a fan mount molded with and integral panel that would close off the open ends of the exchanger... all one piece. Clips on the far end that would support a pull fan optionally.
Maybe I just need to buy another 212 and start fabricating...