What temps are you seeing at load?
To determine if its a cooler issue or airflow issue, follow the following steps:
Pull the side of your case, blow a house fan on high, into your case.
Run your benchmarks, etc, noting temps.
If you see cooler temps at load than before, you have a case airflow issue, not a cooler issue...you should then look at either a better case and/or fans to address airflow. If temps remain the same, you have adequate airflow and should address the cooler.
If you have poor case airflow and buy a new GPU cooler, you'll still be seeing higher temps, anyway, until you address the case airflow issue.