jochen,
I had about the same results you have when I first bought my 96 extended cab new. I first chaged out the very restrictive exhaust system and put a turbo boost and Exhaust temp gauge on it. I then bought a Manual Turbo master waste gate control from Heath diesel. I later got his reflas of the ECM done. It's a whole different truck now. I would have installed a intercooler but I have what they call a light duty 2500 (6 lug wheels) and the frame is a little narrower so I can't install the normal intercooler that most install.\
I do have the dual thermopstat crossover (late 96) and 3.73 gears that were stock on the truck.
I also did a minor adjustment to the spring on the radiator fan clutch to make it come in at 200 deg instead of 220.
I pull a 7200lb (dry) bumper hitch type camper up some pretty steep mountains here in NC with no problem. It will get a little warm if I try to run them at 65 so I back it off a little. I origanally could only get about 25 going up Black mountain near Ashville.
I also pull a 8500lb boat which runs out to around 11k with the tripple axle trailer and don't have any problems on the interstates here in NC. I don't climb mountains though.
96 6.5 ext cab C2500 short bed, 2 IP rep. under warranty. Came with Dual Thermostats & HO water pump,Tweaked fan clutch engages @200, Kennedy exhaust, Heath Turbo Boost, Heath ECM
John Deere Tractor (It likes cetane improvers also)diesel65j@netscape.net (not always monitored)