Options are migrated once on update, and then the migration function doesn’t run again. So downgrading and updating wouldn’t work. It would have to be a fresh 1.4.3 install updating to 1.5.
That’s it then. That’s what I did on the live site – I upgraded to 1.5, saw the problem with masonry, downgraded to 1.4.3 and then upgraded again to 1.5.1. Problem solved 🙂