Well, working backwards from 245 kW, that's 61 kWh in 15 minutes. Take charging losses of 8%, that's 56 kWh. For that to be 80%, we're talking 70 kWh usable capacity in the pack. They can limit the usable capacity in the pack to 70 kWh out of a 95 kWh battery and achieve it. It's still 2.6C charging, but about 26% buffer in the pack.
Not saying this is how they are doing it, just saying this is how they can do it.
We really don't know what Porsche means when they talk about 310 miles of range. They announced this back in Sept, 2015, so certainly it wasn't likely to be WLTP range. Most likely this is NEDC range given its Europe and the 2015 timing. So a straight conversion would mean about 220 miles of EPA range. Well, as long as the Taycan isn't as inefficient as the Audi E-tron, 70 kWh can deliver 220 miles of range.
Note that according to C&D article here:
We Drive a Mission E Concept: A Porsche EV in the Making | News | Car and Driver
600 hp = 448 kW. With a 95 kWh pack, that' s 4.7C discharge which is well within norms.
The Mission-E Cross Turismo Concept car right now weighs 5,720 pounds and they are targeting a curb weight of 4,400 lbs. And the concept car has about 500 hp, not the eventual 600 hp. My conjecture is that they are currently using a NMC 111 pack that is also a bit smaller in size than the eventual 95 kWh production pack.
With 270 Wh/kg cells, Porsche is likely using NMC 811 cells in the production Taycan. That has a lot of cell degradation thus far. They might be able to sort that out in time for late 2019/early 2020. The Tesla semi is likely to use some variant of this cell chemistry also in that time period. With a large buffer, Porsche can hide the cell degradation for quite some time, much like GM's engineering choice with the Volt pack if the cell degradation is still high when the time comes.
What is interesting to note is that the base starting price car... roughly $85,000 will have the smaller 80 kWh pack. With about 26% buffer, that would have about 60 kWh of usable capacity and about 300 kW or 400 hp of power. That also means peak charging is about 208 kW. If the 310 mile range figure is NEDC, then the base model of the Taycan has somewhere around 185 miles of EPA range. And this is the model that competes against the Tesla Model 3 Performance - roughly the same pack size, roughly the same power output, roughly in the same price range. It charges almost twice as fast, but has about 40% less range. But the 0-60 time of the base car is likely closer to the Model 3 LR AWD as it is likely much heavier.