Battery densities are going towards > 500 wh/kg. There's some talk of batteries of several kw/kg long term. And since we're talking technology that is very much in the early stages of development (see the helpful image in the article), that would be an apples to apples comparison. 500wh/kg is basically a done deal. Several battery companies have announced products that are shipping in the next 2-3 years. From there to 1kw/kg seems very feasible. Several companies have hinted at that being a goal for them.