FWIW, I have swapped pistons to something MUCH heavier and ran the engine without balancing. And many others run race engines that way too. I'm not saying balancing isn't a good thing to do, but I've ran engines just fine without it.
The other issue is that when balancing there is actually only a very small window in which any balance job is perfectly dynamically balanced. Something I wonder about when you see race engines going to anywhere 48-55% balance when 50% is believed by many to be the absolute perfect number for a V8.