Particularly for a masthead rig, unless you race I'm kind of thinking the expense wouldn't be worth the performance gain one might
realize. There are many, many other things of greater importance: Properly adjusted rigging, good sails and good running rigging (sheets, blocks, vang, outhaul, etc.) are far more important, IMO.
What an adjustable backstay is going to do for you on a masthead rig is to allow you to tighten the forestay (not sure how much that matters with a furler) and perhaps
bend the mast a bit--perhaps
allowing you to flatten the main a bit. The only time you're really going to care about that is going to wind in heavier airs, and, again, only if you're looking for those last bits of performance improvement.
Don't get me wrong: I sincerely hope I can either rescue our backstay adjuster or replace it. But I'm not going to cry (much
) if neither is possible.