I don't want to make an argument with this point since it may be very driver dependent and possibly even depend on whether Vista or 7 is in use (I read a suggestion in Vista you could easily revert to XPDM on some WDDM drivers by disabling the driver in control panel, gaining gdi perf for some apps).
On 7 SP1 with Nvidia driver (I tried few different versions) the result I got from each of:
net stop uxsms
net stop "Themes"
& even trying to disable the driver as suggested somewhere, was that the GDI benchmark perf dropped.
My initial assumption was that the GDI perf would go up by trying these measures since the candy was turned off.