I've never had this materia before to be honest so I was wondering how much more experience it gives you in a battle? And also, does having more than one of the Exp plus materia equipped on one person give you even more experience after a battle?

Oh and just a side question, when you have a HP plus materia equipped, does your HP go up alot more on each level up? Or is it just my imagination?

IIRC Exp gain is doe by percentages, at base level I think you get 10% gain, at mastered level I think its 50 or 100% gain. I can't see why having 2 or 3 Exp plus materias wouldn't give an increased boost either.

Hp plus increases Hp by percentages too, 10% - 50% so if you have a mastered HP Plus, then yes at level up you would see a greater increase, because your Hp goes up twice, once from the level and once from the materia.

very useful that materia.got everyone to Lvl 99 in no time using it