I have a downloaded Nimrod aircraft that is very good indeed but appears to suffer from a couple of things that may well interact:
1. thrust is limited on all 4 engines to 93%
2. it is only possible to climb the aircraft to 32,000 ft which I would have thought low for a Nimrod.
After 32k feet the rate of ascent drops dramatically and the speed starts to fall off quite quickly.
I lack data on the Nimrod and don't have any information in the software about operating the aircraft, it is possible others do. I would be interested to know if other folk have also found the Nimrod as I have.