Speaking from my experience with ordering Paths of Glory which was on GMT's P500 I know that they don't go to print until at least 700 orders. When I placed my order for POG the count was somewhere in the upper 600's but yet it still took them several months before they printed the game.
I like the P500 system. It is a good idea, however GMT needs to work on their execution and communication. I'll probably never go the P500 route again unless I hear that they've dramatically improved their P500 process.