The pack voltage starts to sag as the state of charge (SOC) drops. As the voltage sags it takes more and more current to get the rated performance from the motor. At some point, the car's electronics will start limiting performance to avoid damaging the battery; there was a thread a while back reporting a power limitation was imposed at 30 miles remaining. Will you still see a 4.4 sec 0-60 time down to that SOC? I don't know, but I kind of doubt it. For one thing, the extra current needed will tend to overheat the battery pack sooner, rather than later. Cottonwood started an interesting thread about his experience at the track; bottom line is that you want to be at the highest possible SOC for best performance.
Behavior when getting low on range
Colorado Track Day - November 3, 2012