My Solution for the known ALT-TAB Issue when performance stuttering and/or loosing framerate:
Use the fucntion 'Force maximum performance clocks' and set this to 'Enabled'.
You can find this function in your systray nextto your clock.
Right Click on your ATI icon > Select your card (mine is 4870x2) > 3d Settings > Force maximum performance clocks
What does it do:
2D applications and 3D applications operating in windowed mode run at 2D clock speeds, which are typically lower than the clock speeds used for full-screen 3D applications. Using the Force Maximum Performance Clocks option, you can force all applications to run at current 3D clock speeds in order to maximize graphics performance.
Note: This feature is available only for ATI Radeon HD 4000 series and later graphics cards.
Works here since my windows is also using 3d clocks now but not sure for all you folks out there. Good Luck!