All these cards have open non directional airflow heat traps. I have 280x's and take the covers off (4 screws for asus) and this allows your fans to work in a case or open rig. The warranty is only for removing the heatsink and this should have no effect other than easy way to keep cool and clear dust.
WARNING - The fans are exposed and you need to be careful they don't hit anything. I lost a couple fan blades but was able to repair them with industrial super glue. 3200 rpm and they are fine :-)
The covers act as a shroud to spread air over fins, you need the covers to help cooling. Leave the shrouds on.