Disabling Aero Makes Computer Slower

A buddy of mine insists that disabling Aero (Desktop Window Management / DWM.EXE) will slow your system down (lets assume it has a mid level graphic card).His logic is that Aero offloads some of the work to your GPU. Disabling it will increase CPU usage. My logic is that DWM.EXE uses a "significant" amount of CPU, regardless of what the GPU is doing. DWM.EXE on most systems I've seen is usually at the top of the list when I go to task manager and sort by "CPU TIME". It uses more CPU time then my browser, more then explorer, etc etc.So how can disabling it increase CPU usage? Where is this change reflected (which process shows increase CPU usage if you disable DWM)? How would you prove or disprove this?