From the 60+ page MacRumours forum thread about the issue, a lot of people don't have the issue at all in clamshell mode (even without the SwitchResX hackery).

From the 60+ page MacRumours forum thread about the issue, a lot of people don't have the issue at all in clamshell mode (even without the SwitchResX hackery). My understanding of the issue is that the card has variable memory clocks to save power. However, to avoid visual distortion/tearing, the clocks can only be changed during the monitor's v-blank. However, when you have multiple monitors, presumably you would need extra circuitry or at least some mechanism ensure each monitor is in sync, or to detect when the blanking intervals match when using monitors with different refresh rates. I don't have a strong knowledge of this sort of thing, so I don't know how exactly this is achieved, but in this case AMD has "solved" the problem by simply running the memory clocks at full tilt 100% of the time, thereby avoiding the need to precisely time changes in speed.