AUSUM mode helps take the card to another level which is exactly what AMD wanted to do with the Dual BIOS switch. For the first time we're really seeing the switch used in a way we wanted it to be used.
Outside of the extra performance, though, the heat does indeed go up another level, as does the power draw. The heat might be a bit of a turn off for some users with it tipping almost 90c, but others won't mind so much. The good news is that the main reason we're seeing the extra heat is because the fan is spinning no faster.
What that means is if you want to jump into something like Afterburner you could add an extra couple of % to the fan speed to help bring the heat down while bumping noise levels up only slightly. It's not a bad tradeoff for the extra performance.
So should AMD have just released the card at 880MHz on the core and again made the switch do very little like the HD 6950 and HD 6970? I personally feel the answer to that is no. The slightly lower clock has helped keep the temperature at a really good level with one core coming in just over 80c and the other in the 70c mark.
In the end, though, I think the most important question is, do you make the switch to AUSUM mode when you buy the card? Like above, I would probably say no. Yeah, the extra performance is nice, but most are going to find it's unnecessary. You can see that even on our 4.2GHz 980x we hit a limitation in some areas.
The AUSUM switch comes in handier for overclockers. But why overclock when we say that you don't even need the extra performance that the AUSUM switch offers? Well, some people just have the need for speed and others are going to want to overclock even higher for some benchmarking fun.
If you're happy with the out of the box performance and you prefer not to deal with the extra heat, leave the card alone. If you want more performance, though, and you don't mind the heat, make the switch. Either way, I don't think you'll be disappointed with what the Sapphire HD 6990 4GB is able to offer you.