Where the "intelligence" of TRAINERS is put to the test!
First of all, I've decided that we are going to make seprate entries for Pokémon who evolve by trading if the tiering placement is different. For example, if Elekid that stops at Electabuzz is considered High Tier and Elekid that fully evolves into Electivire through trading is also considered High Tier, they will be combined into one entry. The opposite would apply to, for example, Shelmet and Accelgor. Accelgor is clearly better than Shelmet.
So, when it comes to your suggestion for Onix atsync, I think you should reconsider its placement a bit. Even if it is for Onix with a trade to Steelix. Metal Coat can't be found until after the main game, where it is available in Clay Tunnel and the Black City shop. The only way to get Metal Coat within the main game is to find a Magnemite holding it, and that's only a 5% chance. I haven't used Onix or Steelix myself, but based on that fact alone, and that Onix is rare in itself, I would see it as a Low Tier Pokémon at best. Bottom if it's w/o trading.
Having said that, I should say that I did almost put it in Low because of its rarity and its lack of speed as a Steelix (I didn't find its low Attack as an Onix to be that big of a deal), but I figured that since its Steel typing pretty much provides an autowin against a large number of gym leader/elite 4 pokemon (and unlike Skarmory, Steelix can actually be an offensive pokemon), Mid could be justified.
I'm not changing my placement for Onix yet. I'd rather see what others think from testing it. If other people think it should be Low I'll change it.