Agreed. Will have to disappoint the thread when I upgrade in a few years though, as I won't be staying with Audi unfortunately!
Overall, yes the speakers are similar. However, the A5 is based on the A4 and shares largely the same speaker configuration by default, and this extends to the B&O system. The A6 is in a completely different league from the A4 (as I'm sure you can attest for) and has a larger amount of speakers, more expensive components, higher wattage, and more advanced tech going into it (such as the omnidirectional speakers which raise out of the dash). This is also why the A6 and A7 have similar pricing on B&O, as the platform is very similar. The A8 is another step up from the A6/7, and B&O on the A8 costs well over 100 grand, about 130 if I'm not mistaken... will make sure quickly.
EDIT: Also 85 grand on the A8, but I see that on the new model it comes standard with BOSE sound which would lower the cost on upgrade to B&O. I do remember it being over 100 on the previous generation however.