Got a certified brand new battery? If not, then with the high resource usage of GB plus the age of the battery, you're going to have to live with that pretty much, as GB is much more resource intensive than FroYo and esp. Eclair were. Coupled with that is the processor requirements - 800 MHz is damned near a 50% overclock of the original stock clock of the Eclair stock ROMs and a 33% OC from the upped 600 MHz setting for FroYo.
So, yes, your display may be eating your battery - but partly b/c the resources that GB consumes when the display is on and partly because of the increased clock cycles that the ROM requires you to have in order to run it.
now that that is said, your best bet is to turn off the display whenever it is not in use, and set the timeout to a ridiculously low level (like 15 or 30 seconds).