IEEE 802.15.4 is a popular choice for MAC/PHY protocols in low power and low data rate wireless sensor networks. In this paper, we suggest several modifications to beaconless IEEE 802.15.4 MAC operation and evaluate their impact on the performance via stochastic modeling and simulations. We found that the utility of these modifications is strongly dependent on the traffic load on the network. Accordingly, we make recommendations regarding how these modifications should be used in view of the prevalent traffic load on the network.