Switching the side fan to pull cool air in is a decent idea...you can add a filter if you wish, but it will drop your CFM by causing restriction. I'd say just flip it and see what kind of results you get, first.
I was thinking on replacing all of the fans, and upgrading two of them to a 140mm and 200mm...
To be honest, this is likely the best route to take and think of including push/pull fans for the radiator on the LCS cooler as well. Improving airflow is always a good step toward any cooler performing better, regardless if it is air, water or some form of combination.