I think it largely depends on how skilled the surgeon is with performing the Z plasty technique. It really is an art from what I hear. I've seen truly incredible transformation with difficult scars (like those on the forehead or neck) that have been camouflaged very well after Z plasty surgery was done.
I guess my recommendation would be to speak with a plastic surgeon that specializes in scar revisions and have your scar explained and analyzed thoroughly (whether it's this surgeon or someone else). Don't go to a general plastic surgeon that tries to do it all. For this you will need one that definitely specializes in Z plasty!