When testing individual stat impact, you need to make sure that the control has as close to the same amount of non-tested stats as possible. If you can achieve that, test away. A margin of ~53 is more than healthy enough.
I hadn't noticed Steel Tassets, but I don't really see a reason to recommend them. Too much of an in-between measure -- if survivability is an issue, Peisteskin Hunting Belt wins because you can attach tier 4 materia to it. If you're shooting for maximum contribution to DPS, Penance wins.
