[Gerusarus]

Coming from a technical and systems design background, I can share the following with you.

You will never need the speed of a USB3 interface for audio or video transfer, unless you are going to be transferring HD and 5.1 surround in the car. The latter are not yet being catered for in any Beemir as far as I know. (Not yet, BMW please take note, hehe)

Some of the technical data.
Data transfer (RAW)
USB3 - 5gbps
USB2 - 1.5, 12 or 480mbps

Bus power required
USB3 - 900ma
USB2 - 500ma

Bus clock
USB3 – spread spectrum
USB2 – fixed spectrum

Looking at the above, USB3 will be a total overkill.