Nelson-class speed was not just an engine problem

Would replacing the Nelson-class engines earlier have been worthwhile as a route to higher speed?

Not by itself; hull proportions limited the gain, and Rodney's exceptional chase performance came with very heavy fuel consumption.

The class's speed reflected hull form and length-to-beam ratio as well as machinery. Captain Dalrymple-Hamilton could run Rodney's existing plant aggressively during the Bismarck chase because replacement was already anticipated. Rodney and King George V then emerged with remarkably low fuel states. Since treaty standard displacement excluded fuel and reserve boiler water, the evidence points toward forced-draft operation and high consumption rather than a treaty cap on bunker capacity.