Yes, credit goes to Deep Thought for the Mew lead, however, I have found that 220 Speed and a Jolly nature is somewhat wasted, as there is nothing to outrun. Lead Groudon has no hope of outrunning a Mew, and Mew can't even dream of outspeeding Darkrai, Deoxys-f, Deoxys-e, or Shaymin (I could go on). There's just not that many leads in that Speed Tier. Therefore, I have gone with an Adamant nature and minimal Speed EV's (around 60 Speed, to outpace Rock Polish Groudon before it boosts) and pouring the rest into Attack. This makes for a much more powerful Explosion (always nice) and still outpaces the things it needs to.
Now, about why I play Ubers. For one thing, I have always preferred to use the best possible things available to me. I always thought "But so and so does the job so much better, too bad he's banned." (Fun fact, Groudon has the same base Defense as Skarmory, and lots more HP) Why not use the best? As a child growing up playing Pokemon, catching Legendaries was always an exciting event, as the battle music clearly denoted: This Pokemon is Special. Watching Mewtwo returns and his ridiculous psychic powers made me practically salivate about finally catching Mewtwo, and the mystique that surrounded the King of Pokemon. Besides, ever since Generation II, the whole reason of buying a Pokemon game was the Box Legendary (Lugia for Silver, Ho-oh for Gold, etc). Sure, you could get Lugia in Gold, and Ho-oh in Silver, but you didn't get Aeroblast and Sacred Fire, respectively, which is the whole point of getting Lugia/Ho-oh. (For me as a kid, at least) Finally, as a competitive player, Ubers takes much more skill, and is much more fast-paced, and there are no banned Pokemon (screw you, Arceus<--he's not even that good, just ridiculously versatile), and less limitations is a better thing in my book.
As a bonus for you all (for reading this wall of text), a little set I was going to keep secret, but as I am not laddering intensively anymore (I got to #9 on the Leaderboard), here it is.
Kyogre@Leftovers
Modest (+SpAttk, -Attk)
252 Hp, 36 Spe, 220 SpAttk
Calm Mind
Surf
Ice Beam
Substitute
coupled with
Latias@Soul Dew (Really, what else would it hold?)
Calm (+SpDef, -Attk*) Edit: Yes, it is -Attk, don't know what I was thinking
252 Hp, 252 Def, 4 SpAttk
Draco Meteor
Reflect
Light Screen
Healing Wish
The magic of this is that, in my experience, after Kyogre takes out Palkia (by subbing on the switch, Calm Minding as the sub is broken, and 2HKO'ing Palkia, which it can no longer do to you) the opponents really can't deal with a Sub-CM Kyogre anymore. However, it is left at around 30-40% HP, and is more or less neutered. But with Dual Screens and full health (and no status if Palkia manged to Paralyze you with Thunder), few teans are capable of standing up to Kyogre again. Some of you may wonder why Soul Dew is on Latias instead of Light Clay, but Kyogre doesn't need Dual Screens for that long, and Soul Dew makes Draco Meteor hit a lot harder, and makes Latias incredibly hard to kill. With Dual Screens and 101 Subs, few things can stand in the way of a Kyogre with even one Calm Mind, which it will most certainly get.
If you've actually read to here, I salute you.