I have scanned a few reviews and the upgrade looks very promising speed wise:
https://www.youtube.com/watch?v=pVA-WZWdEpY
Only downside i have found is that the changed port layout will mean 'old' enclosures are most likely redundant and will need modifying or replacing - bummer if you bought a posh aluminium one.
The ethernet port (for instance) has moved to the opposite side of the board which will mean making space for the higher usb set.
Not seen much yet about any IQ boost for audio though -I never really found much difference between RPi 2 type and the 3 model I had to be honest so not expecting much of a leap.
Also curious how much software upgrading the new model will require and whether the usual developers (Volumio, Moode, Roon etc) have already responded.