I've tried this cooling configuration on my Antminer yesterday.

At first I was getting 49/50C with stock fan off, and 44/45C with stock fan @ 2300 RPM.

Then I tried to move additional fans about an inch away from heat sinks, and it improved the numbers considerably: 44/45C w/o stock fan, and 41/41C @ 2000 stock fan RPM.

I'm thinking of replacing the stock fan with a quieter and slower one, as in this inverted configuration it basically only cools down those big rectangular resistors (marked R47). And maybe add some kind of cover to direct the airflow from the replacing fan along the board.

EDIT: All figures are for 400MHz.