AIB cards all have 24 ROPs enabled, not just the reference 16. http://www.techpowerup.com/143753/AMD-Gaffe-Has-AIBs-Ship-Radeon-HD-6790-with-Too-Many-ROPs.html
Frankly if you bought a HD6790 in the first place, you already got the wrong end of the stick... So much to choose from ATM and IMO the 6790 just is the worst choice...
Update: We have discussed this with AMD and it looks like this was our mistake and the cards really have 16 ROPs.