A primary goal of terrestrial photovoltaics is to reach grid cost parity. This is a difficult task, largely due to the seemingly inverse relationship between efficiency and cost. Multi-junction III/V solar cells on an active Si substrate may break this relationship by providing high efficiency with much lower material cost than traditional concentrator solar cells. This paper employs device models in the analysis of short- and long-term performance goals and expectations for this material system. The modeling results presented here indicate that the III/V on active Si technology may be anticipated to achieve nearly 48% efficiency at 600 suns concentration in a long-term, high volume manufacturing regime and almost 39% in the short-term.