The Plaid can only output its maximum power at 100% state-of-charge (or close to it).

As state-of-charge decreases, so does the overall battery pack voltage. Since the motors can only pull some peak number of amps, doing so at lower pack voltage will always deliver less total power.

I own a Model S Plaid and I used to pay close attention to the OBD-II data, out of curiosity.