You're a developer, so can you explain how some games look the exact same at 1440x900 and 2880x1800, yet the performance doubles at the lower resolution? Valve games all look the exact same at those two resolutions, yet Blizzard games don't. It's quite strange.
It's likely (I don't make either title you mention) down to the windowing mode used in the code.
You have two ways of drawing to the screen the "new" way and the "old" way. Apple are pushing everyone to move over the new way as it support fancy stuff like the notifications and achievement popups. The old way is on the way out and will slowly be phased out I guess.
When playing a game you have two resolutions:
The resolution your monitor (and OS) is running in this is in almost every case the native resolution of the monitor.
The resolution the game is running in, this is usually lower than your native screen resolution.
The "old" way
When the screen goes black and you enter full screen the game will tell the OS to lower it's resolution so it matches the resolution you selected to play the game in. This is the same as going into System Preferences and lowering it yourself. Everything gets bigger and more blurry but your Mac will only be drawing the smaller number of pixels for example 1024x768.
This means the graphics card has a lot less pixels to keep in memory and will run faster.
You can usually tell this mode as when you tab in and out of the game you might see OS X blue for a second or see some resizing really quickly.
The 'new' way
When the screen goes black and you enter full screen the OS does not change resolution and stays at native resolution. The game is rendered at the resolution you selected in the game onto an off screen buffer. Think of it as an image of a single frame of the game at the resolution you selected. This frame is then taken by the OS and scaled up to fit the screen resolution. Lowering the resolution means you have less calculations to do to create the final frame but that final frame is then scaled up to fit native resolution by the OS. This means the card has a little more overhead and will in some cases run a little slower.
This new way as the final frame is in a buffer allows the OS to overlay messages like notifications etc over the top of a game, use Expose etc
I don't know for sure in your case if this is part of the reason but how exactly you render the game does have an effect on the performance at various resolutions.
Edwin