The understanding of reactivity controlled compression ignition has greatly improved throughout recent years. While its potential for extremely low NOx and soot emissions has been confirmed numerous times, there remain some ambiguities about the ability of this concept to attain very high efficiency. Now that increasingly stringent CO2 regulations come into effect worldwide, it is particularly the efficiency aspect that is of great interest. This work combines a heat release study with regression analysis to identify the influence of combustion parameters on the heat release rate shape and the relation it has with efficiency and associated loss channels. A metric called burn ratio is introduced as the ratio of burn durations after and before the centroid of combustion. The burn ratio is found to correlate well to fuel reactivity stratification, and generally characterizes the heat release rate shape. Further regression analysis revealed a synergy between fuel reactivity stratification and the intake manifold pressure. Heavily boosted conditions and relatively early diesel injections result in a notable suppression of heat transfer losses, although some stratification remains necessary to find a balance between heat loss and combustion efficiency. Specifically, this balance is essential to achieve high gross indicated efficiency, for which the burn ratio is finally used to optimize fuel injection.