There's no way I'm being bent over to the tune of £40 just for 8gb.
I agree that is a painful price jump; and is one of the reasons I changed from the Google Play order to Currys... factoring in shipping and discounts, the 16GB is only about £14 more than the 8GB from Google Play.
The problem is while it can support USB audio-out, it's still up to the manufacturer to enable it at a root level, and even then some devices seem to work and others don't.
It appears that the Nexus 7 does support USB audio-out - http://www.engadget.com/2012/06/30/gear4-speaker-dock-supports-usb-audio-for-jelly-bean-at-google-i/