I've read some less than stellar reviews of the Asus. Issues with the DL DVI connector not functioning and poorly fitted heatsink. It could have been issues with the review samples only, but it's still concerning to me.
TechPowerUp managed 1135/1635 (core/mem) with their review sample of the Vapor-X on stock voltage. This gave it a very real boost in performance putting it above stock 7970's and 680's in BF3. A game that nVidia benches real well in.
Edit: You could have problems crossfiring the Vapor-X as well. While it doesn't take up a full 3 slots, it does use more than 2. You'll need 3 slot spacing on your mobo to crossfire it.