There appears to be no advantage for VW to push biodiesel, and although they may be conservative with their 5% number, there is no good reason for you to use more and that may have something to do with VW’s number.
Biodiesel is expensive; my last 5 gallon buy last autumn cost over 8 dollars per gallon, presently about twice the cost of petroleum diesel. My guess is that the same arguments about corn ethanol’s effect on food cost apply to biodiesel. 5% biodiesel does an excellent job of ensuring adequate lubrication for your injection pump if the fuel vendor falls short of adding the necessary lubrication additive to ULSD which is why I use it, one quart per tank fill, usually about 12 gallons.
Here is a post found on TDIClub.com that you may find interesting as I did as I still run an old IDI VW diesel. http://forums.tdiclub.com/showthread.php?t=405766&highlight=biodiesel
Injection pump pressure in a TDI diesel is approx. twice that of an IDI diesel so 5%biodiesel might be good for your TDI diesel.