I'm using 3 mm 1x19 as diamonds, I'm using 390 mm spreaders.
Greg prefers 3 mm dyform. Taipans use 350 mm spreaders I believe.
My analysis on diamond wires was pretty basic. I knew strength was not an issue for these, stiffness is. The stiffer the wires the less sensitive the bending of the mast at the spreaders is to different mainsheet loads. I felt 4 mm 1x19 was overkill but now I'm not so sure. With 3mm 1x19 I have to keep an eye on maintaining proper bend. I wouldn't mind some more stiffness (at the spreader height). I feel it is good to have a stiff middle section so you can flatten the top without losing to much draft low in the sail as a result of the increased sheet and downhaul loads.
But this area is still relatively new to me and I've haven't been able to come up with a dependable model of what happens there.
Good luck !
Wouter