Since this game's a guilty pleasure of mine, I felt I might as well add a few more details as to what I know to the pile of info here. I don't have a DL-legal playthrough handy to provide updated stats or anything since I can't be fucked to replay BtB again without using stat boosters and it's not an especially interesting cast anyway.
On magic and acquisition levels of it
There is an awful lot of postgame magic in this game for no good reason, yes. The levels one would have to reach to actually learn anything are extremely high compared to what is actually necessary to clear the game (the total level of 40 given above is quite high itself; I'd peg a reasonable non-grinding run's endgame level closer to around 35, based on admittedly fuzzy memory) - some of the cast, in fact, don't learn their final spells until as late as a total level in the mid-high 50s; if you get that high, you were grinding for days.
On promotion
20 is the minimum level for class change; in my experience, it's unlikely you'll actually get there at that low of a level (at least for the non-late joiners) without significant encounter avoidance. On runs where I haven't actively grinded but have fought every random in my path along the way, I recall usually getting to Discipline Island with the core early party (Finn/Annie/Edward/Samson) at around level 22 or so.
There is a little bit of consideration in the actual optimal promotion level. When a PC promotes, their level is reset to 1 (like FE promotions, their stats are unchanged). The promoted class has its own EXP-to-level table, which in all cases follows the base class's EXP table precisely starting from level 16. (In other words, the amount of EXP for a PC to get from promoted level 1 to promoted level 2 is exactly the same as for the same PC to get from unpromoted level 16 to unpromoted level 17, and this pattern continues all the way down the table; so level gain is always quickened a bit by promoting, relative to the levels immediately preceding the promotion.) The second consideration is that every PC has some spells that they cannot learn until they promote, so if you level too high before promoting, I believe the PC's learning of these spells may be delayed accordingly; I'll test this with Gameshark sometime. (Spell learning is based on the number of total levels gained. As a general rule, this applies to any spell learned after a total level of more than 30, so one definitely should not level past 30 before promoting.) The third consideration is that most PCs tend to get their best stat growths in the 20-30 level range (notable exception: Samson, he gets his best gains early), from what I've seen, and post-promotion levelup gains follow the same table as pre-promotion gains; in other words, the average total stat gain from level 1-10 unpromoted and the average total stat gain from level 1-10 promoted for any given PC is identical. All of this combines to basically say that optimally the player should promote somewhere in the 25-30 range (25 is probably "reasonable" to a point, 30 is definitely deep in grinding territory, and higher than 30 is bad). Finn is forced to promote to progress the plot beyond Bandore, of course.
PC join levels, and how they matter, if at all
Finn, Annie, Samson, Edward, and Tont all have an initial level of 1. Annie, Edward and Samson all join early enough that their EXP gap is meaningless within an hour of gameplay or so. Tont's gap is more meaningful (assuming he's recruited at the earliest opportunity, the core four have about a 5000 EXP lead on him), but Tont also has the fastest leveling rate of the cast (tied with Edward). By endgame, his EXP gap is not significant, as it is a fraction of a single level by then.
The EXP gap, then, is much more notable for the likes of Domino and Lorele. Domino starts out around 50,000-55,000 EXP behind (join level 15, and it is impossible for the party to be that low at the point he joins unless you run from every encounter ever) and has the slowest leveling rate (tied with Annie). Even at endgame levels, this means he's going to be just under three levels behind Annie, and about three and a half behind Edward if he stays in play. Lorele, on the other hand, joins immediately after Finn's promotion, at level 19, and shares his leveling rate. Since Finn very likely gains no EXP on his promotion quest (running from everything is the only sane way to handle that dungeon), her gap is a little smaller than Domino's; I'd say she safely falls in the range of two levels behind.
Percy
Percy is a special case here, because he's gone from your party for basically the entire game, and when he rejoins, he's been promoted for you.
The way his level works is actually very simple: the game takes his stats at the time he left your party early in the game, levels him up 19 times, promotes him, then levels him up 2 more times after that. So, simply put, Percy's promotion level is whatever his level at the time he left was plus 19 - usually this works out to 23, in my experience, but it can easily become 24 with only minimal grinding - and he rejoins at level 3 promoted. Needless to say, his EXP gap is massive, and for all practical intents and purposes, he is a poor PC in-game, since he mostly shares Finn's equipment (which means he only gets second-best of everything) and, without some focused grinding, is going to be 7-8 levels behind the rest of the party, joining immediately before the fairly difficult final dungeon.
Stats
It's not known whether IQ and Luck do anything at all, as far as I'm aware. The manual is correct that mashing buttons raises the chances of double attacks, counters, and criticals (this is easily observable - the game will make a "ping!" noise when you get one via mashing), but it's not a 100% thing.
