One of the advantages of graphics cards like the Radeon HD 6790, which are based on GPUs with only some of their units enabled, is that you can sometimes unlock the remaining units and get a free speed boost. Unfortunately, it doesn't sound like that feat will be achievable with the 6790.
The folks at X-bit labs brought up the subject with AMD's Evan Groenke, who responded, "It is not something that should be possible on the 6790. Those SIMDs are fused off." That probably means the extra bits and pieces in the Radeon HD 6790's Barts GPU are disabled at the hardware level, which would preclude a traditional BIOS-based unlock—or any kind of unlock, for that matter.
Of course, as we pointed out in our review, the Radeon HD 6790 is barely any cheaper than the Radeon HD 6850, which is based on a faster version of the Barts GPU with more units enabled out of the box. The 6790 starts at $149.99 at Newegg, while the cheapest 6850 costs $164.99, and you can even grab one for $149.99 after a mail-in rebate.