To be fair Lele has no Pixilate Hyper Voice or other things like Will-O-Wisp that Mega Garde has over it along with slightly less special bulk & speed. Lele can hold items and not take the Mega slot in return along with basically been immune to propriety, I think they are equally as good honestly. I do agree that, outside of Trace, Lele completely outclasses normal-Gardevoir in pretty much every ways however.
That being said, Tapu Lele's Psychic under Psychic Terrain is practically as powerful as Mega Gardevoir's Hyper Voice.
Without a boosting item.
252 SpA Tapu Lele Psychic vs. 0 HP / 0 SpD Shaymin in Psychic Terrain: 222-262 (65.1 - 76.8%) -- guaranteed 2HKO (It has the same base stats as Mew, after all)
232 SpA Pixilate Mega Gardevoir Hyper Voice vs. 0 HP / 0 SpD Mew: 226-267 (66.2 - 78.2%) -- guaranteed 2HKO
Of course, Gardevoir has the better Fairy move... but not by much if we add a boosting item.
252 SpA Life Orb Tapu Lele Moonblast vs. 0 HP / 0 SpD Mew: 203-239 (59.5 - 70%) -- guaranteed 2HKO
Mega Gardevoir will have to focus on its support moves and higher Speed if it wants to have the edge, as Tapu Lele's ability to hit harder with one STAB and a little weaker on the other while being immune to priority and not using the Mega Slot...
Provided you don't run
both, of course, overwhelming their own checks. Mega Gardevoir does love Psychic Terrain, getting two stupidly powerful STABs to work with, and preventing a Mega Scizor check. And considering fast Steels essentially don't exist...
232 SpA Mega Gardevoir Psyshock vs. 0 HP / 0 Def Shaymin in Psychic Terrain: 232-274 (68 - 80.3%) -- guaranteed 2HKO
232 SpA Mega Gardevoir Psyshock vs. 4 HP / 252+ Def Eviolite Chansey in Psychic Terrain: 309-364 (48.1 - 56.6%) -- guaranteed 2HKO after Stealth Rock