IMO the differences in acceleration arise from different maximum power that the packs can deliver.
The PEM converts 375V DC from the battery pack to 900A AC 3~ with variable voltage & frequency for the motor. It does this from 0mph until max DC power output is reached, then power stays constant and the amps start to drop.
If the cells in the Tesla pack survive 4C for a few seconds, we get 160kW from the 40kWh pack, 240kW from 60kWh pack, and 340kW for the 85kWh pack. The latter is roughly in line with the 435hp mentioned in the
Performance Features.
The performance drive train will deliver 1200A and outperform the 900A version. I guess that the Performance can reach maximum acceleration of 10m/s² whereas the standard drive train reaches 7.5m/s².
The power P required to sustain acceleration a=7.5m/s² of a car with mass m=2000kg is P=m*v*a. Solving for P=165kW I get v=11m/s = 25mph. Then, the base model might be 140kg lighter for the reduced number of cells, improving the result by 7%.
I conclude that the Model S with 40kWh pack will
keep pace with 60kWh and 85Wh pack
up until 25mph, then start to lag behind. The 60kWh model will keep full acceleration up to 16m/s=36mph and the 85kWh will reach 340kW at 23m/s=50mph. These are all speeds where power to overcome rolling resistance and aerodynamic drag is <15kW so we can neglect this.