During the cleaning process they (the dealer) was to install a port in the side of the turbo. . This port was to be used for later cleaning if it was deemed necessary... There is a good tech bulletin on this.
The dealer now offers a chemical in a can, (you need 6) that you connect to the threaded port and follow the instructions...
One of the key issues here is to keep the exhaust brake on all the time. . the brake forces the use of the ring inside the turbo and will help prevent build up... as every time the exhaust brake comes on, the ring has to turn, thus knocking off the soot...
BTW - my 08 5500 now with 110K miles has been great... . but we drive it like we stole it...