I had the chance to closely compare a 2 x 9 Sora front derailler with a 2 x 9 Tiagra. They appear to be identical except for the finish on the die cast portion; the Tiagra is polished, the Sora dull. My postage scale, good to 0.1 ounce, could detect no difference. I surmise that the on-bike performance of these two would probably be the same. At what model level do the incremental differences between models become more noticeable in Shimano front deraillers?