Remember, ATI Xenos has a secondary GPU that reduces the load on the main GPU e.g. HDR being done on the secondary GPU.
Not really a secondary GPU, it's a daughter chip of the Xenos.
Many don't consider that mode true HDR precision. The daughter chip is mainly there for cheaper (advertised, even coming as free to developers) anti-aliasing, but of course there are plenty 360 HD games lacking any anti-aliasing. For example Halo 3 or just partial AA like Gears of War, etc.
In reality using the daughter chip involves a lot of data transfer (tiling in higher resolutions), severly limiting this potential due to the small amount of Ram on the daughter chip.