More diesel power would not make Graf Spee a high-speed raider

How fast could Graf Spee become with 80,000 horsepower of more powerful diesel engines?

Probably faster, perhaps around 31 knots, but not proportionately faster because the hull and diesel machinery were designed for long-range cruising rather than high-speed operation.

Extra machinery power still has to overcome the hydrodynamic penalty of forcing a long-range hull much faster. A rise from about 50,000 to 80,000 shaft horsepower may produce roughly 31 knots, but diesels are less forgiving of being pushed beyond their intended operating profile. Graf Spee's hull form was selected for economical long-range cruising, so added shaft horsepower produces diminishing speed returns. Installed power is different from sustainable performance, and machinery alone does not turn a raider into a fast battlecruiser.