To get an accurate estimate at lower levels, you'd need to calculate the average number of Attack Boosts found by that level. At a minimum, assuming that all 3 Attack boosts have been found at low levels skews the results. Due to the way rounding works, at 0 boosts my Mage Plate never results in a lower damage.
I don't think an item that's only useful if you prep it and pick a certain race is a good design. The previous Tri-sword design (+3 per health potion) is a great example of that. It was overpowered due to being balanced to still be useful for other races. Even if I conceded that the Mage Plate was currently balanced for Humans, that just makes is vastly underpowered for other races.
Replacing the escalating penalty (upwards of -50%) with a flat penalty (roughly -20%) provides other possibilities. At worst, it gives us a prep for Orcs other than Sensation Stone.