There are several of us that have noticed the battery life being close to 50% of what it was prior to the epub "upgrade". I don't use an sd card, have the slideshow turned off, have the power save mode turned on, and don't use the epub files on my device (I convert to lrf - epub sucks). I've definitely noticed the battery life being pretty poor after getting my device back.
I think the only hope we have is if Sony offers an update to the new firmware to fix the problem (doubt that'll happen), or if someone can figure out how to hack the new firmware like they've done with the original 500/505/600.
The battery issue has been mentioned on other threads, so you're not alone - but I don't think there's a fix.