ok, lets end the broscience here ;)
I did tests on the biggun's hair model, with 65k strands. Tried different embree subdiv settings, from 1 to 16. Results:
memory usage was always the same. Subdiv settings are applied only during traversal
Performance, in Mrays/s:
Subdiv 1: 5.6
Subdiv 4: 5.2
Subdiv 8: 5.1
Subdiv 16: 4.2
So only going into extremes - single subdiv, or 16 subdivs - have some effect. I am tempted to increase the default subdiv to 8, as there is no peformance penalty - remember that the scene was JUST the haircut, nothing else - with other geometry, the impact would be negligible