There are two types of exhaust brakes used on the 2008 C9 Contessa. If your coach has a diesel particulate filter with a variable geometry turbocharger and a regenerator, the PRXB exhaust brake is not normally recommended.
The other type uses a conventional exhaust brake (Pacbrake). If this is what you have, it is very simple to install the new design Pacbrake.
I know of one Contessa owner that has had this done, and he said that even though the braking was improved, especially at lower RPM's, that it was not the major improvement that was advertised. If you keep the RPM's up (as per operating instructions) when using the Pacbrake, you will probably not notice the difference.
Gerald