The ideal stem length for a rider can indeed vary based on their individual style and preferences. A more aggressive rider might prefer a shorter stem for quicker handling, while a rider seeking stability might opt for a longer one. However, as you mentioned, there's a trade-off between comfort and performance.
A study by Cobb et al. (2006) found that a stem length of 120mm resulted in the lowest aerodynamic drag, but a 100mm stem allowed for a more aerodynamic position. This suggests that there's a balance to strike between the two factors.
As for hard numbers, it's difficult to provide a definitive answer. However, a general rule of thumb is that a 10mm change in stem length will result in a 0.5-1 degree change in handlebar height. This can have a noticeable impact on a rider's position and comfort.
Ultimately, experimentation is key. As you mentioned, it's important for riders to try out different stem lengths and see what works best for them. It's a bit like finding the perfect saddle height - there's no one-size-fits-all solution.
What have other riders found in their experiments with stem length? Have you noticed a significant difference in your own riding?