Over the last 18 months or so, I've retro fitted the Harmonic Dampener, the exhaust forward under cover, the exhaust rear undercover, the rear wishbone covers (spoilers) and the extended front engine underside splash shield. I'm not sure if any or all of them contribute to sound reduction, but if I have the artificial noise generator set to Eco at 70mph in the driver profile setup, then my GTI is commendably quiet (even on the not so quiet original Pirelli P6 tyres) at typical motorway speeds. Only those horrible concrete sections of the motorway spoil the quietness (but I guess that would be the same in any car) ?
In fact, I would say it's quieter at motorway speeds than the Audi A4 and Jaguar XF we have as pool cars that i drove recently, and much quieter that the company's BMW X1 pool car.