RS4/RS6/Pheaton Discs are around 360mm and will fit your existing hub, but they are VERY expensive. A more sane option might be the 345mm disks from the B6 S4, as they're available from motorfactors for around 50quid each
Bear in mind that any front upgrades should be accompanied by upgrading the rear by a similar amount or the resulting change in brake bias will actually lengthen your stopping distances.