The folks at X-bit labs confirmed this in talks with AMD's Evan Groenke, who said: "It is not something that should be possible on the 6790. Those SIMDs are fused off." So unlike early versions of the Radeon HD 6950, which could be turned into a 6970 with a simple tweak, it looks like the Radeon HD 6790's Barts GPU is locked at the hardware level.
At least the card is said to be a good overclocker, and with board partners implementing their own cooling solutions users should be able to squeeze some extra performance without much trouble. However, going for an Nvidia GeForce GTX 460 (768MB) at the same price or paying a just few extra bucks for the more powerful Radeon HD 6850 sounds like a better option. The cheapest 6850 costs $165 on Newegg and you can even get it for less after a mail-in rebate.