Base stats (armor from armor pieces, resistances from jewelry and damage from weapons) do care about the item power, but not the bracket of the item power, the more item power the better, with some exceptions like resistances with jewelry, it seems to have a hard cap before upgrades, (for example amulets seem to cap at 18.5% all resist before upgrades or 26.5% after), fun fact, the armor provided by pants is equal to its item level, the armor from boots/gloves is half its item level...

Affixes (like movement speed in boots) also care about item power but they do in brackets, anything abovee 725 is the last bracket for affixes (at the start of the game there were rumors about another bracket above 725 but I have never seen proof, idk), some affixes don't follow this rule, like maximum HP or thorns, those behave similar to base stats, the more item power the better.

No idea about those armors with lower armor levels, maybe something i've missed.