Recent work in sensor network energy optimization has shown that batch-and-send networks can significantly reduce network energy consumption. Batch-and-send networks rely on effective batch data transport protocols, but the throughput of state-of-the-art protocols is low. We present conditional immediate transmission, a novel packet forwarding mechanism, with which we achieve a 109 kbit/s raw data throughput over a 6-hop multi-channel 250 kbit/s 802.15.4 network; 97% of the theoretical upper bound. We show that packet copying is the bottleneck in high-throughput packet forwarding and that by moving packet copying off the critical path, we nearly double the end-to-end throughput. Our results can be seen as an upper bound on the achievable throughput over a single-route, multi-channel, multihop 802.15.4 network. While it might be possible to slightly improve our performance, we are sufficiently close to the theoretical upper bound for such work to be of limited value.