Tracing almost always requires post editing, so on the face if it you may be asking for a bit more than a tracer can offer. Also it's worth noting that before you trace, you might be better off if you prepare an image for tracing, keeping your intended result in mind. EG: Increase contrast and sharpen edges . Also try using Mehdi's Fine threshold before you send it to Xara's trace.

FInally, why not give VectorMagic's tracer a try? It may bring you results closer to what you seek