Though headstay sag opens the slot, it also makes the sail fuller, and will backwind the main (it is more obvious on a monohull without battens). You could put negative luff curve in the sail to counter this, but it may be to flat for other conditions.
Comparing two boats with all things basically equal the boat with the straighter headstay will point higher.

Again, a historian from the Tornado class could share their wisdom on this one as they have worked it for years.