Here's my biggest issue,....

Certain armor, even though it has two materia slots, caps the stat (like Control) at 2 or 3, marking it in RED .

Doesn't that mean that you will only get a +2 stat increase instead of 3 or 4.
With this premise in mind, why do people do forbidden melds of extra stats when it's already capped.

I saw a guide on how to "overmeld," but i really don't understand the concept. I also saw someone meld +vitality on a tanking ring x5, when the ring capped the first meld. Doesn't that mean the other 4 materia slots are wasted on stat that is capped?

Any clarification/explanation on materia caps, overmelding/forbidden melding and how this fits into optimizing Crafting gear to do ** Synths is GREATLY appreciated.