You are forgetting some important facts. Espeon is less physically bulky compared to Xatu, and Xatu has an immunity to Groudons strongest move. Where are your calcs about Espeon stalling Groudon nicely?
4 Atk Groudon Earthquake vs. 252 HP / 0 Def Espeon: 232-274 (69.46 - 82.03%) -- guaranteed 2HKO
You can't morning sun reliably and you can't reflect, Groudon will KO if you come in on Earthquake (which is usually the case). If you decide to go impish max defense you are still less bulky than Xatu in the physcial department minus the eq resist as well. You also miss out on the speed, which you mentioned as a quality over Xatu. It is simply outclassed as a screener. Don't use it as you do in OU. Espeon has offensive qualities, like the 2HKO it gets with grass knot on Groudon, hurting Dialga (getting it in range for other attacks to kill it, therfore fullfilling its job of keeping SR away) a bit, not letting Ho-oh sub (thanks to psyshock) and stopping all set-up sweepers if its sash is intact.
Wish? You are never going to pull that off. And if you use a moveslot for that, you will be forced to 1) run morning sun, and therfore using up 2 moveslots on healing moves 2) protect to make wish reliable. Neither of these options are good, because if you add screens as well you are set up fodder for Ho-oh and E-killer for example.
Heal bell doesn't give it a niche either, just because it can cure others of status doesn't mean it's worth it. Most often, it must pay with its life to heal bell.