The turbo is really small on the 3.0 Mercedes motor. It looks like 260 HP is about the limit.
As far as transmission tuning, they came with the same transmission NAG1, as the SRT-8 Jeeps. I believe the NAG1 also came in a few Chargers, Magnums, etc. There is quite a bit of performance/hotrod following for the SRT-8's so that means there are lots of transmission options. One of the more common names I hear is "Southern Hot Rod."
My wife is the proud owner of an 07' Jeep Grand Cherokee CRD so I keep my eyes and ears open for performance upgrades. If I lived in place without emissions laws and testing, I'd be really tempted to add a second turbo, get some larger injector nozzles cut, pay GDE for some custom tuning, install a built transmission, and go run low 12's at the local drag strip.
Completely stock, these torquey diesel Jeeps will bark all four tires from a dead stop when you turn the traction control off and build a little boost against the brakes.