Nelson and Rodney engine-refit limits

How much speed would Nelson and Rodney gain from a 1936 engine update refit?

Without another shaft, an engine and boiler refit might raise practical standard speed to about 26 knots, but not remove the ships' underlying machinery limits.

A 1936 machinery refit would be useful but bounded by ship architecture. Without another shaft, roughly 26 knots is a practical standard speed, and normal operating power remains distinct from the maximum output an engine might briefly survive.