It's all about wattage, not just voltage, so of course it depends what resistance atties you'll be using on your ProVari, but honestly, any battery will drain faster than a 2.8v regulated battery... What resistance are your atomizers ?
Anyway, if you want more battery life in a small form, I suggest you look at variable voltage eGo-class batteries (eGo-C Twist, eVod vv, iTaste vv 3). There are many good ones on the market, some, like the Vision Spinner, come with 1300 mAh capacity. And you can adjust the voltage to your liking. The iTaste MVP is 2600 mAh--but it's box shaped.