Spreader deflection/mast stiffness

I don't really go with that theory. Try looking at it this way. If you have a mast with shrouds but no spreaders as the wind loads up the sail, the topmast will bend to leeward. When this happens the mast between the hounds and deck level will bend to windward, with the mast forming a smooth arc. If you now put a set of spreaders on they restrict the movement of the mast under the hounds to move to windward. This has the effect of reducing the movement of the tip of the mast to leeward. As you increase the length of the spreader it pushes more on the mast thus increasing the stiffening effect on the upper mast section. If this is overdone you can get an S- shape develop in the mast sideways, which normally means that you have overdone it on spreader length.  (Note Just because Adam Bowers, allegedly, once said that " increasing spreader length doesn't stiffen up the mast sideways" doesn't make it right!)

That was my original comment on not overdoing it on the the spreader length or you will get the mast moving to leeward at the spreaders which is not fast. But generally the relationship between spreader length and sideways stiffness of the rig is well established.