Piscean,
I have been following this thread since it started the other day and I thought I might chime in as well.
I know all too well what it's like to wrap your mind around a problem that's really annoying me. One of the things that I have found most of the time is that I spent most of my time thinking so hard on the complex logicals of a problem that I sometimes overlook the simple truths.
Ask yourself this; Have you ever breathed 100% O2 before? Say, in a hospital? If your line of reasoning is correct, you should be able to slow your breathing down to 25% of your normal rate while maintaining normal activity since you're used to getting only 21% normally. Your breathing didn't rate didn't change much did it? Nor will it. Here's why, (physicians in the house help me out here if I miss anything or get something wrong.)
Your body is only capable of using so much O2. Now what that level is depends heavilly on your body and what demands you are placing on it. You read in that link you posted (I read it too), the breathing reflex is usually triggered by elevated levels of CO2 in your body. So, think about this logically, if your body can really only use a max of say, 21% O2 (at a resting state), you could breathe 100% O2 (at a resting state) and your body will still produce the same levels of CO2 as you were before and hence triggering your breathing reflex at the same intervals.
Now that's just the extreme basics of the biology of breathing. Take into account the psychological side of breathing and we'll have another 50 page thread in here.
Cheers!