I wonder where they're getting 315 miles of range from in the P100D. The P90D has 270 miles of range, or 3 miles/kWh. For 100kWh it should be 300 miles of range. At 315 miles of range the P100D is 5% more efficient than the P90D.
If the upcoming 100D is also 5% more efficient than the current 90D, that car would get somewhere around 343 miles of range.
4.5 miles/kWh is hugely optimistic, and a 30kWh reserve is highly unrealistic. Tesla's range numbers are based on roughly 3.3 miles/kWh (which is reasonably realistic) and the floor is maybe 5kWh.
I can't explain the discrepancy, but it's definitely not just the floor. Either they've made some sort of efficiency improvement, or the difference between the two battery packs is more than 10kWh and they've just rounded off the figures.
If the upcoming 100D is also 5% more efficient than the current 90D, that car would get somewhere around 343 miles of range.