@ Step last: You say 'space constrained' just to clarify, is there enough space to install a taller cooler? All of the aftermarket coolers will require at least two slots of space.
As an alternative: First remove and clean the existing cooler and replace the thermal compound with a premium grade material.
What case is it installed in? Perhaps you'll get improved results by adding a front or side mounted intake fan to improve airflow to the card cooler. A good test to see if limited airflow is a/the problem is to run the system with the side panel off and see how the temperatures respond.