On the same engine with everything the same, the 20" case Super Turbos measured about 2 dBA at idle and 3 to 5 dBa less at higher engine speeds, all at static conditions. Under load is slightly different but the 20" case version is most definitely quieter and less prone to drone.
Even though they are quieter, they allow as much air to pass through due to the longer internal sections that enable the air to smooth out after the flow reversal (my theory anyway).
Tom