I'm not actually participating in this research week but I have used Mew before. It's far and far worse than last generation, where it was arguably the best Baton Passer (with a set of something like Rock Polish / Swords Dance / Taunt / Baton Pass, with Taunt blocking any attempts of phazing and status and Rock Polish and Swords Dance boosting for a Groudon / Rayquaza sweep. Unfortunately that is far worse than last generation: Dragon Tail works through Taunt. If you decide to use Substitute to block Dragon Tail you get ruined by Roar. Roar of your own might work to block both, but you'll leave yourself open to status.
Another big thing is Shell Smash. Mew is no longer the best quickpasser in the game, Smeargle and Gorebyss are. Smeargle has Spore and only needs one turn to set up for a devestating sweep, where Mew needs a lot more.
As for Celebi, has anyone tried a Perish Song variant? It seems pretty good on stall teams to me, being the Perish Song user which prevents you from being taken down by last Pokemon setuppers such as Cro-Ogre and Bulk Up Dialga, while also being able to rack up entry hazard damage quite nicely.