I've personally always run Modest on Mega Gardevoir since it's outsped by a majority of the meta anyway. I've been interested in finding a bulkier spread for it that can still outpace some notable threats and give it more longevity, but to no avail up to now. In the end, it's all a matter of preference and team needs. Teams with fast Pokémon appreciate the power that Modest gives, while others like Timid to outpace a few threats after Mega Evolution.
Generally, I believe Mega Gardevoir is still as great as ever, since its Pixilate Hyper Voice remains one of the best nukes in OU available and it's gained a slew of new Pokémon to check: special Mega Altaria, Mega Gallade, unboosted/+1 Mega Slowbro and, most notably, Mega Sableye and Mega Latias. The mere presence of Mega Gardevoir is frightening, as it can Taunt any Pokémon bar Mega Sabes and even set up with Calm Mind. It's solid as ever.