In my case I worked at the Electrical garage at FCA. So testing this stuff was nothing new to me. My stuff was voltage drop checked it front to back under load. Charging was checked at the battery itself. Alternator was upgraded (75 amp). Though its still to small. Think about it. Cooling between rounds with big fans and your water pump. 3-5 starts per round going up to staging. Then a few minutes at most with the alternator running mostly at idle or low rpm. It doesn't have time to recharge what you are taking out. Tried adding a heat shield on the starter. Pulled the starter apart and tested all components. Tried a different starter. Tried 3 different pairs of batteries. I watched guys with low compression motors, small motors who have zero issue. But when the engine is big and the compression is up there, different story. Now in the late rounds the battery(s) are no longer at the fully charged 12.6 volts. They're down to maybe 12.2volts. 12.2 is roughly 50% state of charge. Resistance goes up with heat. Now your starter needs every bit of voltage it can get. The Voltage is what gets the job done.
Doug