Have found that my preference is just before the onset of 2nd Crack, between 10 and 20 seconds before. I use a slightly modified popper with speed control over the fan and glass chimney.
I really enjoy the Yirgacheffe and love the exotic characteristics it adds to blends. As a SO, it is very intense with a very prominent top end and that ever mentioned, "winey" flavour. After initial cupping, I havent really persisted drinking it as an SO instead, prefering to blend it, especially with my Colombian Pennsilvania, PNG Sigri and Indian Tiger Mountain. It just adds an extra dimension to the flavour profile of these beans that is absolutely fantastic.
Maybe someone else can comment on what they have discovered in roasting this bean as a SO espresso, since I am more into blending my espressos.