There has been some discussion on the Bendix King user group about alternate batteries to standard alkaline AA batteries when using a BK clamshell. For years I have kept a set of nine 2000mAh Sanyo Eneloop batteries in my clamshell and I will say they outperform the standard BK battery as far as run time and there is also a notable improvement in "getting out" from areas where I have been unable to raise dispatch or hit a repeater. Even in my FT-60r "alkaline tray" I have used the eneloops with results that outperform the standard Yaesu battery.
The Energizer Ultimate Lithium are some of the lightest and longest lasting AA batteries on the market. They have a shelf life of over a decade even in extreme environments, so you can leave them sitting in your backpack for years and don't have to worry about them. Here is a spec sheet from Energizer comparing them to Alkaline cells. While the lithium's are not an economical alternative to the eneloops, which have a +1000 cycle recharge life, when backpacking the lithium batteries would cut the weight of my clamshell in half. You could probably go on a three day backpacking trip with just a clamshell of lithium cells and as long as you keep transmissions to a minimum it would probably last the entire trek.