I cannot stress to you all enough how important it is to balance an engine when replacing pistons. You say factory built engines were never perfect? Could be why some ran better and lasted longer. Why anyone that does engines would downplay balancing and say its not necessary is beyond me.
Balancing is not only important for power & rpm's but also for endurance.
. I taught my son that balancing was the most important thing to do on anything that spins when he was 10 years old building slot cars. He would balance everything.