I think the idea of a higher buy-in game monthly is a bad idea. Bear in mind that everyone is playing for leaderboard points, not just hard cash. So to have a game that excludes those who cannot afford a bigger buy-in means that the playing field is no longer level, as they would not be able to play for the points in that game. I"m sure this is not what APAT would want. $20 is a good number, most players can afford it, and the prizes are tasty enough when you get to the business end.
The idea of having different levels for different bankrolls is a nice concept, but it"s just overcomplicating things. The current setup seems to be working just fine IMO.