For those interesting in seeing how battery Ir effects voltage drop in your battery I threw together a spreadsheet at 3 different current draws.
In summary most packs at room temp are around .006 ohms. If charging a pack at 40 amps can drop the Ir to .003 ohms this would provide an additional 0.030 volts @ 10amp draw to your motor over an entire run which in 17.5 TC would equate to about 70rpm on the straight.
The bigger story is punch out of the corner. An Ir of .003 @ 30 amp draw would provide an additional 0.090 volts to your motor over a battery with an Ir of .006 ohms.
When you’re a top level driver at a big race this could be what gets you a TQ. There are other factors as well, Motor winding resistance, Motor Rotor strength, car setup, tires, etc that would negate this advantage but that's what competitive racing is all about. Putting all the puzzle pieces together so you’re the fastest person on the track. This is why the same person does not win all the time, one of these pieces is missing when someone else has it all together.
I have to say reading this thread has been a bit of an eye opener for me just like it was learning about rotor strength at Nats this year.