Would five shafts make Nelson or Rodney faster?

What speed could a five-shaft Nelson or Rodney reach?

Probably no more than a four-shaft version, about 30 knots, because hull form would become the limiting factor.

A fifth shaft would probably add little beyond a four-shaft arrangement. Accommodating even four shaft lines would require some hull lengthening, which changes the length-to-beam ratio and makes the hull more efficient in its own right. Once resistance from the existing hull form becomes the governing limit, another shaft cannot convert all additional power into useful speed. The estimate therefore stays at no more than about 30 knots rather than treating shaft count as an unlimited route to performance.