There is a direct correlation between engine operating temp, condition of the thermostat and emissions/fuel economy.
This is true on gasoline engines, which are homogeneous-charge, air-throttled designs, and target an operating point as close to stoichimetric as possible. Diesel engines are fuel-throttled, and by design run well lean of peak-EGT, stoichimetric, and even peak power due to their stratified-charge nature (overall stoich is rich-as-fuck at the combustion site). A diesel will blowing black smoke and melting holes in its pistons before it ever comes close to stoich, much less rich. A (relatively) richer running diesel engine will be making more power, not less.
A diesel running cold with an open thermostat won't necessarily have it's fuel ratios affected so much as its injection timing. Injection timing absolutely does have an effect on emissions and fuel economy at any one given fueling/power-output level.