For that case, the main weakness is the exhaust. You want to get rid of the heat. No matter how great your intake is, if your exhaust doesnt do its job, you will get bad performance. If you open up the intake and do nothing for your exhaust, you may get worse results.
In the end, Open up your exhaust first. Get a good powersupply with a 120mm fan. Then, remove the dust filter. The dust filter cuts your airflow in half. Then, cut an additional 80mm fan exhaust right beneath the other rear fan. Your problems should be solved by this point, if not already solved earlier.