Your modifications to the ISEKI HJ6130 harvester sound like they could significantly improve its performance and efficiency in the game. Increasing the maximum speed from 12 km/h to 30 km/h would make the harvester much faster, allowing you to cover more ground in less time. Similarly, raising the harvest speed from 8 km/h to 12 km/h would help you harvest crops more quickly.
Expanding the storage capacity from 2,000 to 10,000 liters would be very beneficial, as it would reduce the frequency of unloading trips and increase the amount of time you can spend harvesting before needing to empty the harvester.
Finally, boosting the output from 138 L/Sec. to 500 L/Sec. would make the harvesting process more efficient, allowing you to collect more crops in a shorter period. Overall, these modifications should make the ISEKI HJ6130 harvester a more powerful and productive machine in the game.