In that case, is there any way I can try to argue Shaymin-S into OU? I can see a few points that could make it easily counterable, so I'd like to try and convince someone.
Not really, it outspeeds pretty much everything, sets up on whatever it wants. As such a fast subseeder, it can outsall stuff like Blissey, while grass types which resist leech seed are easily taken out by Air Slash. Scizor's Bullet Punch is a good check but loses to any Skymin with HP Fire (surprisingly common). Serene Grace and Air Slash means it can flinch through everything, and if Seed Flare gets a SpD drop, even stuff like Heatran will be flinched to death if it lacks Earth Power.
There's a reason it got 100% of the vote to be banned. It was stupidly overpowered - amazing stat distribution, great speed, a movepool which is just enough to let it beat anything, a fantastic signature move and one of the best abilities in the game with two STAB moves which exploit it (Seed Flare and Air Slash).