Given a mast head rig a backstay adjuster isn't likely to bend the mast atall.
I don't know as that's strictly true. We have (or had
) a backstay adjuster on our Pearson 30, which is a masthead rig. While we didn't get near the bend you'll see on many fractional rigs, we definitely got bend. Was it enough to flatten the main enough to make much of a difference (if any)? Don't know. Was just getting to the point of experimenting with that.