Yes and no. The pixel shader still has to be run for every pixel covered by the impostor, because you don't know in advance if the custom depth will be in front

Yes and no. The pixel shader still has to be run for every pixel covered by the impostor, because you don't know in advance if the custom depth will be in front of or behind the z-prepass. So you likely pay the full rendering price for an impostor, even if it completely invisible.