I had a chat to my dealer last week, apparently VW's engineers have come out and said that they are unable to improve much on the T5 design (hence T5GP and T6) as it is pretty much as good a design as they can get. Other than drivetrain and some consumer electronics upgrades along the way. Its really only customer experience improvements otherwise.

The shape apparently is ideal and the space is optimised, 70% of the T6 has new panels, though that would be hard to pick. Multivan for instance, the rear seat can now slide without having to be folded down! Big improvement if you ask anybody who owns(owned) as pre T6 Multivan.