A wild guess on my part: perhaps output level at a cooler 32.8 Hg is similar to output on a hotter 35.3. Not that you aren't entitled by the gods to any extra power, but for the FAA, STC's seem to be based on keeping risk factors and tolerances at or lower than factory spec. You get improved efficiency, engine life, etc., from the cooler temps, but leveraging them to get HP above the factory spec was perhaps considered unapprovable, or more difficult to certify, and thus the placard.
Again, just a guess. But then again, maybe Dan was hinting at the same thing, on second read.
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