Hey Folks.

I will probably word this incredibly poorly. Apologies in advance.

Does the level of a sub-persona matter, other than any increases in HP and SP that may bring? As far as I can tell, HP and SP are the only stats a sub-persona have. No Strength, Magic etc.

Would sticking with a sub-persona with useful skills, but a level lower that the party member, be a problem?

That wouldn't be a problem. What I do is keep a persona with useful skills in until it learns all of it's skills, then I fuse it into something else and have the new persona inherit the good skills. That way you're still getting higher level personas with better HP & SP and eventually you'll get better skills.

From what I've heard Sub-persona level matters a lot in the calculations. There is a penalty for using personas lower than your character. I believe it starts at a 5 level difference with a 10% reduction in all of your damage and healing numbers. I know at maximum it hits a 50% penalty but I don't know when you hit that threshold. Its a good idea to keep your personas up to date even if that means losing skills. In this game raw numbers mean a lot more than skills unlike regular persona games.

They also get new spells every few levels. What I've been doing (just wrapping up the first floor of the second dungeon) is stick with a sub-persona for a little bit until I can fuse it into something better, bring over the spells I like and use and keep it rolling from there. Choosing which spells you want to carry over when you fuse is a real game changer.

Pretty much this. This has actually been the hardest thing to adjust to coming from the core Persona games. I feel a lot weaker in this game than in Persona 3 or Persona 4. I just had my first wipe because I was fighting 3 identical enemies, which knew the equivalent of Wind Break and Magarula (medium wind to all). In this game, that Wind Break skill makes your characters weak to wind, so when the other two used magarula, it wiped my entire party in one turn. This game is definitely more about leveling until you can withstand enemy attacks than it is about the typical "Knock everything down, then All-Out Attack" that the other Persona games were.

With that being said, my strategy thus far has been to keep the same party members for the entire game and fuse more powerful sub-Personas, ideally with Resist *insert elemental weakness here* and some elemental skills, because the primary Personas learn next to nothing at super infrequent intervals. My advice is to not get too attached to any one Persona/group of skills, because you need to constantly be willing to fuse them into more powerful Personas.