For comparison, here is a pic of the engine bay of the new 2012 E63, which gets a 5.5L V8 turbo. So, 1.1L more than BMW, although even with the Performance Package, power output will be no more than the M5, while peak torque at 590ft-lb will probably be slightly more than the M5. In base form, however, it is only 518hp and 516ft-lb.
Now we look forward to the next RS6, which will get a 4.0L V8 turbo rumored to be 550hp+. Throw in the XF-R with 5L supercharged V8 and this has the makings for one hell of a four-way comparison test.
Actually, we could throw in the CTS-V too and make it five.