Overstocking will equate to more polluted water. Ammonia, nitrite, and nitrate are 3 of hundreds of pollutants in tank water. These other hundreds of pollutants are not easy to measure, so there are no hobby test kits for them. I treat my pets as if they are my kid. I wouldn't want my kid to live in a polluted area (air, land, water) because its bad for their growth, development, and lead to disease.
The best indicator of the total amount of pollution in tank water is by looking at nitrate. The higher the nitrate, the higher amount of total pollution. If you end up using any products or plants to decrease nitrate, then you are masking the actual amount of nitrate (total amount of pollution) in your water.
Overstocked tanks needs more maintenance and at least 50% water changes 1+ times weekly depending on nitrate levels. This is easy to do in the short-term, but is not something most hobbyist would want to do long-term.