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Looks like your 2 stop under was pretty good. the most Red I found was 243. Makes me wonder if 2 1/2 stops under might be a strategy for Sunny 16 red's.
The color channels that you see in the final rendered image are somewhat of a red herring. They have already been multiplied/mixed by the necessary white balancing, so they don't represent the headroom available in the "real" data. In particular, a red value of <255 might already be badly clipped --- the RAW reconstruction algorithm is using residual signals from the other color channels to guess how far above clipping the red actually is. Google for "unitary white balance" to learn how to get closer to the "real" histogram that will tell you about color clipping. Canon sensors tend to produce higher internal RAW values for the red channel, so it clips much sooner that you might expect; hence the advantage of heavy under-exposure, followed by pulling the levels back up in post, for imaging red-heavy subjects (in this particular example, I boosted +0.8 stops in post after underexposing by 2 to produce the image shown).