every phone acts differently to ROM/kernal combos. but you should see an improvement over stock with a ROM and kernal combo. Toggling 4g will be a HUGE battery saver. Wi-Fi is also a huge battery saver. it really depends on your usage on how long the battery will last. i can usually get about 12 hours of constant 4g (no toggling) i do some light game playing, forum browsing, some music streaming, messaging, very few calls.
it will be close, you can at least try it out for 14 days and if it doesn't work for you, return it.
the phone is great, and pretty much on par with other LTE devices on the market (minus the razr maxx) in terms of battery life. i have a car charger, a work charger, and a home charger so battery is no concern for me.
also, i have heard some great things about JuiceDefender. it turns off data when screen is off, but comes on from time to time to sync mail and other services you have syncing constantly. i am hearing double the battery life...but like i said, battery life is not an issue for me so i have never tried it out myself.