I don't think Tesla is concerned with maximizing margins. They have achieved a very nice margin, and I think they are keen to maintain it, but I think we need to look at everything they do from now on in terms of Model X and Model 3. For instance, the dual motor Model S is Tesla's way of proving out the dual motor architecture and software that will be used in Model X. This de-risks the Model X launch since that's one less "new thing" in Model X.
In that light, my guess is that the 70D battery is a next generation battery that is intended for Model X. I don't know if that means a new cell or what, but this is probably another piece of Model X that is being launched early. Maybe their production capability for this new battery is only enough to support the demand for entry-level Model S right now. Once it spins up a bit more they'll replace the 85kwh car with a 100kwh version (or something like that). Then, they'll launch Model X in 70D and 100D versions having already worked out the kinks in pack production.