Martinsliquidlab did testing on this but since archived on a new site may be difficult to find. We still carry a lot of mindsets over form last millenium practices but the thing is hi SP fans are really a thing of the past except in rare instances. Back in the day 30 fpi fans were common, now we see mostly 8 - 12 fpi stuff. In addition, especially w/ Intel / nVidia stuff, we are not seeing the temperature wall be the stopping block nowadays ... for the most part, when I am OCing, I am hitting voltage levels that make me skittish before I hit temp levels that I am concerned with.
Pull generally performs better up to about 1500 rpm ... push is better above 1750 rpm.
"It was very clear that when using two fans per radiator section, that the radiator intself acts as a flow spreader and provided great benefit to straightening out the air for the second fan in a push/pull configuration. Just like doubling up with a pump, there is a pressure benefit to doubling up on fans and this translates to about a 20-30% performance gain. The added fan pressure simply means and added gain in air flow and corresponding performance. None of the experimental pull/pull or push/push configurations provided any noteworthy benefit as it seams the air is simply too disturbed from the first fan to allow the second to perform properly when air is moving."
I couldn't find the article I was looking for where he addressed your issue but the search feature of the new site isn't very fruitful.