A guy in a german Forum tested a couple of Sandwich and Push/Pushpull combos.
He tested with tripple 120mm Slim Radiators from Magicool and heated the water up with 300W.
The results were:
3 Fans Push
800rpm 14,4° K
1200rpm 11° K
6 Fans Push/Pull
800rpm 11,8° K
1200rpm 9,2° K
So the advantage in his case was about 15-24% but I don´t think that your system is producing 300W of heat. On the other hand your Radiator is a lot thicker and might profit more from pushpull.
I still wouldn´t expect more than 20% advantage and thats not that much.
If anyone wants the link to the post send me a PM. It is in German but the Pictures are self explaining.