Basically, depending on item level and slot each piece of gear has a cap on how high each stat can go. This cap is individual to each cap (so hitting the cap on one doesn't stop you from raising other stats), but it also INCLUDES THE BASE STATS. For HQ battle items, the stats that are already on it will be at or near the cap. Hence why putting Mind on there is actually a bad idea.

You can tell if you've hit the cap, because when melding materia it will say how much the materia will increase the stat. If you hit the cap, that number will be lower than what's listed on the materia, and I believe it's also a different color.