I just searched and found there are quite a few topics discussing armor by the piece. So far I looked at two, and neither seemed to address my piecemeal question.
Just a bit earlier I was looking at the section about customizing armor by the piece, and one thing raised a question: rounding of decimal maneuver penalties. Why even bother listing them as decimals when it's just going to go up or down to the nearest number?
For example, I have a pregen warrior mage who has a soft leather shirt, rigid leather bracers, and rigid leather boots. Before I'd read that bit, I assumed that her total minimum maneuver penalty would have been -1 rather than -2.
The other thing I've noticed, is that a previous group was right in one way about getting shafted with armor by the piece. A full suit of soft leather has a CP of +2 PP, whereas if you piece that same suit together it has a much higher CP.