I think you're confusing energy and power density. Going from 2.4ah to 4.8ah doesn't (necessarily) mean you get twice the current from the cells, it probably means that you get the same current for twice as long. With current technology, you can get high power/low energy cells or vice versa, but not both. My point is that the Roadster needs the number of cells that it has to generate the power to get the acceleration (and presumably to absorb the power from the regen breaking). Unless the power density of the high energy cells, or the energy density of the high power cells gets better, you're stuck with the current cell count or lower performance (or, I suppose lower range if you switch to high power density cells). When the Tesla folks were in Seattle last fall, someone asked them about building a Roadster with a half-sized battery pack (and so lower weight & higher acceleration) for track use. They said that it wouldn't work because they need all the cells they have for power, so I'm guessing that there's not all that much wiggle room.