On the TDI the intake manifold has a 51mm inlet. Does this mean that intake ducting over 51mm is pointless?

The inlet/outlet on the airbox is 64mm, but I would like to reduce to 51mm ducting to facilitate a tight route (no bends). I assume the larger diameter is to allow for bends in the ducting that narrow the intake.

AlliSport void their warranty on VGT turbos if the intake is restricted, they include snorkels as a restriction. The safari snorkel has a 51mm connection, as does the AlliSport intercooler.... so is 51mm really too small for intake?