I only have the 4 speed auto, but would have to say I have only reached 3500rpm once or twice where I have underestimated a gap, or road conditions have changed. usually I will plug along to a max of 2500.
I would assume that the auto is reacting to your driving, If you bury your foot to the floor, the car will no doubt go to 3500 and beyond, look out the mirror see the 'smoke' thats diesel that you wasted. From my experience the car wants to change gears before 3500, it would only be going beyond that in responce to your driving, try easing back on the accelerator and bingo, gear change.
After nearly 90,000k I would also dispute your claims that the CRDI is underpowered. I came from a 6 cylinder Falcon, and can't say I have ever wanted for power, even when carrying the family and luggage up a steep hill on holidays.
It would seem to me, that you need to search some of the threads on how to drive a diesel, it also seems from your response that you will not do this....