So is there any real, objective reason to use mega gardevoir now that tapu lele exists?

In all seriousness, the only reason i can think
of is the higher base speed. Hyper voice on its own doesn't seem worthy enough to waste my mega slot.

252 SpA Gardevoir-Mega Psychic vs. 0 HP / 0 SpD Manaphy: 177-208 (51.9 - 60.9%) -- guaranteed 2HKO
252 SpA Pixilate Gardevoir-Mega Hyper Voice vs. 0 HP / 0 SpD Manaphy: 211-249 (61.8 - 73%) -- guaranteed 2HKO

252 SpA Twisted Spoon Tapu Lele Psychic vs. 0 HP / 0 SpD Manaphy: 267-315 (78.2 - 92.3%) -- guaranteed 2HKO
252 SpA Tapu Lele Psychic vs. 0 HP / 0 SpD Manaphy: 222-262 (65.1 - 76.8%) -- guaranteed 2HKO
252 SpA Tapu Lele Moonblast vs. 0 HP / 0 SpD Manaphy: 156-184 (45.7 - 53.9%) -- 46.1% chance to 2HKO