From my experience, and that includes some time I spent with QSDG working on ILC 40's with load cells up to an including some more recent time spent on TP 52's and most recently with Alan Andrews working on the Andrews 28 project at the Whidbey Island Race Week, it would not be accurate on a swept spreader fractional rig to suggest that headstay say on a properly tuned rig cannot be significantly adjusted by the backstay.
That is not to suggest that other considerations such as halyard tension, headstay length, rig tension/tunning and mainsheet tension are not also factors in this case but I would not overlook how a properly tuned rig with an effective backstay system can help in heavy airs.
That said I would also submit that jib halyard tension would have a more dramatic effect on sag on a furling equipped boat then one with a foil, for obvious reasons.
