I would have said that the shutter speed will have little effect. I think Manfred's example is not mirroring the effect of what you asked and is a completely different lighting set up ie "[lights] positioned far enough forward ". I would suggest that to darken the background indoors it needs to be kept in the background and use the fall off in light as per inverse square of distance to produce the effect you are after ... coming in close to the subject with the flash on the camera as Manfred's example shows to a limited degree. So I think you are quite dry

edit ... sorry a bit damp

In making this comment I am assuming that the ambient light is quite low, several stops below 'correct exposure'. With a highish level of light your application of syncro-sunlight applies indoors or out. An interesting application I had not thought of.