So the reason Takumi didn't use the paper cup for the race is because he only needs it as a reference point for the uphill to control load shifting with the tofu. He's been using the paper cup on the uphill for 5 years now with Bunta slowly upping the amount of water in it as a way to finetune his control. He doesn't use it on the downhill though because he drives it a lot faster to get home quickly and goes around corners differently since he's not transporting anything.