I looked and the data on the servers does *NOT* match the data that I intended to be in the game.
SO. this armor set will be changing. It will be more like the original poster stated, where each piece is an upgrade.
the visible armor in Underfoot, once IN Underfoot gear, is intended to be a piece by piece upgrade, as well as a set upgrade. This is *NOT* true when going from SoD to Underfoot, but once in underfoot each piece is an upgrade over the previous piece. Going from SoD to Underfoot you do need to look at the armor as a "set" to see an upgrade, and even then it would be possible to pick and choose pieces from SoD to get certain stats to be better than lower tier Underfoot, probably at a sacrifice of something else. This is also *NOT* true for non-visible armor, where you need to be looking more towards sets, and you may not get a piece by piece upgrade, and may even see some weirdness in sets depending on which piece you pick.
From this thread.