I can give you some more concrete info here.

You only get the full stats (defense, mdefense, evasion, etc.) from equipped gear if you meet both the Optimal Rank and Favored Class requirements. Being the wrong class reduces the stats by a flat percentage (I think!), whereas being below the optimal rank reduces it by a percentage that gets smaller the closer you are to the optimal. This means you should almost always wear gear that favors your current class, and you should typically avoid gear that's above your current rank. There are exceptions in both cases, like wearing Mitts on a CON for the bonus MP (doesn't favor CON), or equipping Bronze Plate Mail at R32 on GLA (optimal R38).

There's another good reason to stick to gear of the appropriate rank and class, failing to meet either requirement causes equipment to degrade significantly faster, leading to more frequent repairs. It's pretty bad I hear if you meet neither the class nor rank requirements. Wearing R50 gear at R10 can be very expensive.