I noticed a pretty big difference on battery life on my iPad. I switched back to Safari and noticed a longer lasting battery. I workout in my gym at the house and browse the web while having the iPad hooked up playing music to stereo. I do it on a daily basis for roughly the same amount of time. With Atomic my battery was dropping it down to the mid 70's% during that time and when using the Safari, it would stay in the mid 80's%. I switched back and the Safari kept it in the 80's once again.
I liked the atomic interface better than Safari, mainly the tab usage and finger swiping along with the hot key buttons on the bottom. After 4.2 install the Safari, now has the number of open windows displayed which has helped me with the tab issue that I really liked on the Atomic browser for. I still have Atomic, but I value my battery life later in the day, so I will have to stick with the Safari for now. My assessment was pretty accurate due to the consistency of my pattern and usage when comparing. I did this for a long period of time too. It took me a while to realize why the battery all of sudden had less life.