I totally agree with you on the OU/UU thing. Let's just create a pokemon that we think will be competitive and if it falls into UU, so be it. All this talk about watering down the pokemon is not very practical.
Thing is, we're making all these pokemon with the intention of them being OU-worthy. It's quite unlikely that something that's designed to be OU-level would end up being weak enough for UU even if it wasn't as good as planned. If we could create something that's viable in OU but not overpowered in UU like Tentacruel, that'd be great, but that's much more difficult than creating a clear-cut OU poke. That said, I don't think these polls should focus on the UU metagame, as that's still (comparatively) unexplored compared to OU, and just because OU is the standard metagame.
So far, all the polls (on Syclant and this one) have tended toward the more powerful choices. Some say too powerful, some even say broken. I don't think so. I'll reiterate a point I made earlier -- if I posted Metagross' type, stat line, move pool etc.. BUT removed the name "Metagross", and simply proposed it as a new pokemon --- a LOT of people would yell "IT'S BROKEN!". Obviously, Metagross is not broken. Powerful? Hell yeah. Broken? No way.
I see and understand the point you're making (hell, my sig is a pretty good example of things not working out as expected, albeit in the other direction), though Metagross probably isn't the greatest example (I'd have picked Garchomp and it's 120-bp, STAB, 130 stat hardly-resisted Outrage or Blissey and it's obscene hp/sp def for something that might seem broken in theory but not in practice).
Despite having said that, I do want to be conservative on the stat total, because as has been stated many times this is going to be one hell of a pokemon.
One other thing I'd like to state is that I'd be very hesitant about giving it Shadow Sneak. Depending on how strong this thing is, with Shadow Sneak it might be able to OHKO several of the pokemon that have the ability to truly threaten it (Gengar, Azelf, Alakazam, Medicham), who are rare enough as it is. Of course, that'll depend on it's final stats, but...eh.
Edit: Something with 100 base attack (252/Adamant), +1 attack (Bulk Up) using STAB Shadow Sneak (no boosting item):
134.92% - 158.73% on 4/0 Alakazam
104.96% - 123.66% on 4/0 Gengar
88.55% - 104.20% on 4/0 Medicham
83.90% - 98.63% on 4/0 Azelf
Zam and Gengar are both toast, and if SR is up Azelf has about a 20% chance of survival while Medicham has about a 40% chance of living. Hmm...