This isn't only on the ROM. What kernel you are using, what settings in setCPU you are using, how hot your phone is running, what you do with your phone during those 8 hours all will probably have a bigger effect on your battery life than a ROM. Yes a ROM can help some, but if you are playing a Homerun Battle 3D for an hour then constantly using your phone pretty hard (say constantly browsing the web), checking e-mail, posting on your facebook, then go to see what Lady Gaga is doing on her Twitter account, while streaming her music on Pandora. All of that with 7 screens, live wallpapers, max brightness, and tethering for your coworker's iphone...8 hours isn't too bad. This is a rather powerful device, with a relatively small power supply. This isn't your standard phone. My previous phone could basically do 5 things, place a call, text, e-mail, play games, and set my DVR to record while I was out (technically it could also browse the web, but I had a cheap package on it 25mb). I could go days without charging it. It didn't have this big detailed animated touch screen, multi-tasking abilities...etc
Beyond all of that, you could still change ROMs. Everyone seems to have different experiences with every ROM. Someone will say "X ROM has the greatest battery life ever", then the next person will say "Y ROM has the bestestest." You could try either one, and not end up having a good experience with either for whatever reason. You find that Z ROM works best for you. So in reality your best bet is to just try a few, and see what you think.