@joe
I guess by "net coding gain," you are discounting that part of the gain which comes from concurrently using the much lower bitrate? i.e. compared to if you had already reduced your bitrate by 1000x, and then from there added the 1000x bits of coding, the coding part only brings you an extra 3db? Wow. Considering the really high overhead (1000x more bits), that's not much to get excited about.
Anyhow, as far as home automation goes, if there's a range issue, it seems you're better off keeping the high bitrate (to maintain high battery life on the mote) and just deploying another mains powered, wi-fi+rfm69 gateway node in the vicinity. Am I right?