What they mean by "change the damage calculations of tanks"?

[Tyrial]

Pentamelds are a bit more powerful than that. An i150 accessory has roughly 2/3rd the primary stat of an i210 accessory resulting in pentamelds that are roughly 33% better than the highest ilevel gear available.

This only holds true while you still need the VIT that the pentamelds provide. Once your left side is upgraded sufficiently to no longer need the extra VIT full slaying is superior.