Wasn't questioning that... I met Andy at the L2B... nice bloke...
I'm not saying it's impossible, but that's just over a 62% increase in torque...
Where-as, with your Cummins, another 200 lb/ft with mild tweaks is only a 33% increase, which is more achievable IMHO...
To get to a 1000 ft/lbs, which is a 67% improvement, you need to start changing injectors, bigger turbo's etc etc... But IMHO, a Cummins is probably in a very mild state of tune as standard anyway... mainly due to planned usage and engine longevity.
Ian