If I recall correctly, HP is directly read by the ECM on electronically controlled diesels and torque is calculated. Every diesel engine manufacturer know precisely how much HP is produced per pound of fuel burned for their engines. Since the ECM know how much fuel is injected into the engine it than knows how much HP is being produced.
I'm sure others can clarify/expound on this but that's what I seem to recall.
I've used the Silverleaf to monitor both and have seen 505 hp occasionally and the 1550 torque often.