Other options available are full of LiFe
There are other options available. We can start using the 3.3V batteries that powertools and laptops use. Most of these batteries are LiFe based. The best best would be LiFePO4. A123system makes batteries with up to 60C Burst, but there cell form factor is quite large sized for r/c.
Other companies make batteries in the 18mm x 65mm size or know in the idustry as 18650's with 1100mah most common. The 3.7V LiPO 18650s are reaching 2200mah but can not handle over 5C discharge. 18650 cells are the same diameter as a A sized battery but longer. Two of these in series would be a suitable replacement in 1/18scale but would amount to being the same size as a 4-cell 2/3A in stick form. For 1:12 and larger I would want a different sized battery. I was thinking batteries the same diameter as sub-c but longer, 55mm so users would not be confused if there were NiMH or Life. Getting two of these cells into a 1:12 scale would not be a problem, getting four would be. It would be same volume in cells of as a 5-Cell NIMH pack. Yes I know that 6-cells used to be in 1:12. In Sedan and Oval they would run a 2p2s battery pack.
Just to put this in perspective, running a 6.6V pack would be close to the new Euro standards as 5-cell. It would mean maybe speeds in 1/12 picking up but not as much as running a 7.2V pack.