A wise and comprehensive answer by Kurt! I'll just add a clarification and an additional suggestion.
The Echo Kr8-compatible fat batt has an
automatic battery, like your current Kr8, but unlike most other fat batts (e.g., eGo, Riva, KGo). And it is an
excellent auto batt: completely reliable battery engaging with moderate draw, 3.7V (like most Kr8s and the Riva and KGO, but more than the eGo's 3.2V), and up to 1300 mAh (depending on which version you choose). [The only thing I
don't like about auto batteries is that, by necessity, they are
unsealed. It is unwise to fill your cartomizer without first removing it from the battery. Too often, I forget.

]
If a 380 mAh battery and 2ml capacity cartomizer is a sufficient enough upgrade, I strongly recommend CigEasy's
Elite 808. It is now my favorite PV ... and I've got a lot of them. I enumerated my rationale in my recent
review.
BTW, I
don't think much of "attys and cartridges [or tanks] vs cartos" ... unless you use those attys for direct dripping (i.e., with
out ineffective cartridges or mediocre tanks) or with juice feeder mods. [An exception to the tank dislike are those large tanks that surround a cartomizer, e.g., J-Tank, MAP Tank, Liquinator, or VPX Cartomator, which I've not used, but about which have read/viewed great things.]