No scientific data . but this is my opinion.
1. The pusher is mounted to the front. It is covering some of the surface area of the rad during driving. It doesn't really matter when at a stop since the only air flow is basically that of what the fan can produce.
2. Pushers are usually almost always mounted directly to the rad surface, there is almost no room between the fan blades and the fins of the rad. What i noticed when I had a pusher mounted on mine for a short period of time, was that it is very hard to feel the direction of air flow. I found that alot of air flow seemed to be bouncing off the face of the rad and back out towards the fan. Basically causing it to fight itself.
I tested this out by simply holding a piece of tissue paper in front of fan, in various areas, and the air flow was all over the place, when placed on the engine side of the rad, with the pushing there was some air flow making it threw the rad but it was small .
3. The only air flow at a stop , is that covered by the fan. The surface area of the rest of the rad is getting absolutely zero air flow .
Moving the fan to the other side ( puller) and placing it in a shroud that fits well. Now when the fan is running any air that fan is moving is pulled threw the rad, it literally has no other place to get it from. The shroud increases the the surface area where the air is being pulled from. You are basically making a vacuum .
Using the tissue paper method here, air flow at the front of the rad can clearly be seen, sucking the tissue right up against the rad any where over the finned surface. Placed on the other side, the air flow direction is clear, and even blowing towards the engine bay and assisting in air movement under the hood , too help keep everything else in there cooler as well.