Hi,

Regarding stats, you get somewhat more gain in a stat if you have equips that increase said stat when you gain the level. It's not going to be a point-per-level basis or anything like that, but it does show some difference over time. Besides, you generally want these equips on the characters in question anyway. There's no point to not putting magic on Vivi, strength on Steiner, or spirit on Zidane. This is one reason of many that Ribbons (and early game, Glass Buckles, Extensions, etc.) are often my relics of choice.

Regarding missable items, GameFAQs: Final Fantasy IX (PS) Miss-able Items FAQ by Rebirth Flame Though I honestly don't see the point to bothering with most of them since they're hardly unique or hard to get. Actually, you get most of them just by playing the game and not literally avoiding all the shops or random ! marks on top of your head.