They tested this by running benchmarks with Aero on and off. The conclusion? Areo Glass barely affects performance. They talked to AMD about these results and here's the answer:
FiringSquad: We ran some benchmarks with Aero Glass on and off and noted no performance difference when using Catalyst 7.1! Normally under an OS you see a slight 3-5% performance hit when you turn on the eye candy, but that wasn’t there under Aero Glass which was definitely surprising. Do you think the new driver model helps Aero Glass performance or is it something else?
Andrew Dodd: Basically (pretty much because of the new Vista driver model) the Vista OS just suspends Aero in the background, (once you enter 3D exclusive mode it would be a waste of GPU resources to keep running Aero). So that’s why there’s no performance impact