That's a good drawing. I don't think you'd get the full 8" of stretch because the boltrope is prestretched inside the luff and there is the strength of the sail material added to the boltrope once the downhaul is tightened. Still....even 1" of stretch is a significant amount for a well set up rig to be changed. Wire certainly does stretch a little. One method of rig tuning is to measure the stretch in the shrouds as you tighten the turnbuckles.
Alberg 35: With a philosophical flourish Cato throws himself upon his sword; I quietly take to the ship.