...since now everything 2D related is done in software and there's many more layers of abstraction now.

Actually quite the reverse. Almost all 2D operations (apart from font rendering) are now performed on the graphics card if the graphics card supports it - if you fill a big white rectangle on the screen this is usually done by pumping a parameterised shader to the graphics card to do the coloring whilst DWM gets on with the busy work of double and triple buffering everything to give you funky swishy graphics.

One of the big reasons why there's a perception of slowdown with 2D graphics is that most 3D apps turn off DWM when they become fullscreen, and thus get exclusive access to the graphics card and a massive boost to their thread priority whilst the game is playing (in order to improve performance at the expense of background apps).

Most 2D games however tend to run windowed, and thus have to compete with the rest of the system for resources, including fighting the blur-shaders that DWM is using for desktop compositon (which is how aero glass works).

The basic way that DWM works hasn't changed in Windows8, so don't expect a big change in the next release.