Looking at the static pressure vs. flow for your fan and at some of Bill's old
results at Overclockers, the airflow seem about right.
If read the whole article and compare your rad, you can get a sense of how well yours is performing. For the airflow I'd say it's decent.
I doubt that your rheostat is a problem. Cleaning the rad inside and out might improve the air-water temperature difference a bit, but more air will definitely help.