I have a friend that put all three GS mounts in his 900s. I asked him about the diesel mount as i was wondering about it too. He said it was notably stiffer and worth the money for the upgrade. Just be aware that there is definently more cabin vibration. I got to drive it with the new mounts and it feels so crisp and fresh but it also makes it vibrate like a race car, which I personally like as your more in touch with the car, but many Saab owners probably won't like it.
Put simply, its without a doubt one of the next things im getting for my car.