I had the same questions, a number of months ago, when I first installed my HR-01 Plus in my Antec P182 case.
Test #1
Scythe S-Flex SFF21Fs (both ~1000RPM) in both the top and rear spots, exhausting air out of the case.
Scythe S-Flex SFF21F (~1000RPM) pushing air through the HR-01 Plus.
Seating the HR-01 Plus in a south-north or the usual east-west direction resulted in the same CPU temperatures, with my E8400 @ 3.8GHz.
Test #2
Scythe S-Flex SFF21F (~1000RPM) in just the top spot, exhausting air out of the case. Rear spot closed.
Same CPU heatsink/fan setup.
Seating the HR-01 Plus in a south-north direction resulted in slightly higher system temperature (~2C) than in Test #1. CPU temperature was the same as in Test #1.
Test #3
Scythe S-Flex SFF21F (~1000RPM) in just the rear spot, exhausting air out of the case. Top spot closed.
Same CPU heatsink/fan setup.
Seating the HR-01 Plus in the usual east-west direction results in slightly higher system temperature (~2C) than in Test #1, and equal to Test #2. Cpu temperature was the same as in both Tests #1 and #2.
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Cooling differences: I was expecting slightly better CPU temperatures with two exhaust fans, but that wasn't the case. The only benefit was a lower system temperature. I would expect increasing the speed of the CPU fan would be the only thing that would lower CPU temperature (Duh!). Orientation of the CPU heatsink didn't matter in my case.
Noise differences: Test #1 was the noisiest, with Test #3 being the quietest. The case is on the desk next to me, with the top exhaust fan being at ear level.
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Since the increase in system temperature was minimal with just one exhaust fan, I decided to stick with the complete setup of Test #3. Just one exhaust fan in the rear spot, the top spot closed, and the CPU heatsink/fan in an east-west orientation.
I'd expect results can differ based on your choice of CPU heatsink, amount of case fans and their speeds, and other components you have in your PC (like a video card that doesn't exhaust air out of the case)...