The same thing happens to me when I switch to the high performance chip. The battery life basically cuts in half.
Anyway, I've found that the time remaining isn't entirely accurate because it is based on whatever you are doing in that point in time. For example, I was playing a 3D game through VMWare on battery power, and the warning came up for 10 minutes of battery life left, but I closed the game & VM and lowered the screen brightness a few notches and I was able to squeeze 40 minutes out of it.