The racial doesnt matter, what matters is where the pvp players are. If horde racials are slightly better people will play on horde, this will by itself create a pull for people to go horde (more people = easier to find people to play with). Take retail, everyone played horde on TBC for pvp because the racials where better, and people stuck to horde all the way up to wod (for the US, EU switched sooner) even though humans were clearly better then the rest. But because players were on the Horde side people played horde.
If you nerf the human racial/balance it people will eventually simply swap to whatever race is instead the best race for pvp, which in turn will create a new faction unbalance if it happens to be a horde racial. Again, look at when they finally nerfed emfh in legion, everyone went horde for pvp becase of the racials. The minmaxers/"pros" go first, the rest follows.
This is especially true to warmane since faction changes are basicely free.
There will always be a best racial.