Why have a tiered invite with tiers that would generate less than a full event?
If, for example, you have 500 spots at AmWorlds, and 25% of invited players tend to sign up, the first tier of invites should be 2,000 players. It wouldn't accomplish much to invite 400 people and get 100 entries; then another 400 people and get 100 entries; etc.
Prior to last year, it didn't matter a bit. Either you got an invite and went to Worlds, or didn't get an invite and went to Worlds, if you wanted to go. At the very least, the threshold for invites should be what is likely to fill the event, or a little less.