Dolphin striker wire should be very tight with the boat unrigged. The aim is that the beam does not flex when the load of the mast is applied. Flexing the beam will eventually (or quickly) weaken it. It will also change the alignment of the hulls.
Now completely off topic —- in your reference to the Sydney Heritage fleet - a couple of years ago I organised for a load of Douglas Fir spars to be cut and hauled from Bago State Forest (near Tumut) for the heritage fleet. A selection of logs up to 19mt length. There is a small stand of Douglas Fir trees which were planted as a trial in 1927.
The Soren Larsen is now after a couple of 21 metre spars to replace their main mast. They will weigh about 4 tonnes each.