After I downgrading to Radeon 11.x driver for nearly 5 months, AMD finally released a good enough 12.x 
driver for the FSX. It is also the choice of Samy Fay who recommended the driver in a comment about his crossfire setup a week ago.
I wouldn’t say the new driver boosts the FSX’s performance obviously. Also my observation is subjective. However, my FSX running under this 12.8 version does give me an impression that it is above the overall level of the last 11.12.
FSX TIMES 