What the BBRC officially used to balance the tiers was this:

Tier 1: Any team not in another Tier
Tier 1.5: Chaos Pact, Slann and Underworld
Tier 2: Vampire
Tier 3: Halfling, Goblin, Ogre

For the BBRC meeting last October to finalize the CRP we defined the Tiers like this though:

Tier 1: 55% to 45% win rate in average league play (with ties counting as 1/2 wins)
Tier 1.5: 50% to 40%
Tier 2: 45% to 35%
Tier 3: 35% to 30%

Galak