whose differential gets fitted to these with the PP?
Is it VW's own or something from one of the usual manufacturers?
Their own, but designed and made by Borg-Warner. It's actually
mechanical, but electronically controlled (contrary to Wavetrac,
Quaife, Peloquin). Volkswagen say "VAQ" while Borg-Warner call
it "FXD". I'd highly suggest doing a google search for pictures of
the internals.
If you're aiming for performance, the PP is the right starting point.
Both diff and brakes are great value, whether you buy it new or
used.