Thanks for your responses and insights, mariush and IanB.
The AC3 files I'm compressing are overwhelmingly extracted 5.1 audio streams from my DVD collection or captured broadcasts. A typical bit-rate is 384 kBps.
As for apples-to-apples comparison, the closest thing I can come to is taking a single 5.1 AC3 file of, e.g., a typical 2 hour movie. If I downmix to two channels with NicAudio as part of the AC3 -> WAV step the resulting file with "neroaacenc -lc -q 0.40" will typically be about 35% the size of the original AC3. If do not downmix, but use otherwise identical settings and materials, the resulting AAC file will be about 90% the size of the original AC3.
Obviously these percentages will vary from source to source, but are remarkably consistent within +-5% among the dozens of transcodes I've made the comparison. This consistent differential still seems excessively large.
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