However, pretty much the only energy- and power-dense cells commercially available are in 18650/26650 format. I specifically have information about Tesla's new 21700s but I'm not sure if that's been made public yet. I believe it's public that the energy/power densities are low, as are the costs, but I'm not sure if specific numbers have been released yet.
https://electrek.co/2016/11/02/tesla-panasonic-2170-battery-...
I kid, of course, but this is why I believe that 21700s will probably be lower energy density: a) current 21700s have a low energy density, b) heat is tricky, and c) Musk wants to make cheap cells, and optimizing the last few Wh/kg to push him over the edge is expensive.
I do concede it's certainly possible they'll be over 310 Wh/kg, but I find it unlikely.
I think that a very likely outcome is that when the cells are released, we find out that we're both right. For example, if they have a density of over 310 Wh/kg... at 0.1C discharge or something similarly low. But at 1C or 5C or 10C, the energy density falls to around 200 Wh/kg.
[0] Sanyo's cells are the current leaders, about 230 Wh/kg. With 18650s the leaders are between 280 and 310 Wh/kg depending on how you measure capacity (cutoff voltage, discharge rate, cooling, etc.)