In other words, the old modules shouldn't say X characters of Y to Z level, but rather, X characters of Y XP, presuming of course, that the game actually does balance that way, which I'm not sure that it does.
Doesn't matter if it does, as, in classic games, purely mathematical balance is not necessary.
But just a point about your assumption: Given that, up to 9th level, classic thieves tend to need about half the XP of fighters, and given that XP requirements approximately double each level, a thief will lead a fighter by only a single level most of the time.
Oh yes! From a design perspective it sure is. At the beginning of the campaign, a player selects his character's class - if the DM doesn't enforce the roll 3d6 six times, right down the values in the order given on the character sheet and see which class you might select - and is stuck with it for the life of this character. The campaign may run a long time, but the player has no chance to correct his decision. This is an example of extremely bad design!
Old school is, in part, about making choices that matter. A "bad" choice of class IS (relatively) permanent... if it were not, the choice would not have mattered.
When all classes and races are perfectly mathematically balanced, and in combat a magic-user, fighter, cleric, sorceror, thief, etc. all have about the same effectiveness (at a given level or XP total), then the choice of class
did not matter. But in a proper old-school game like S&W, you have choices that matter. So you're playing a 1st level magic-user, and your spell for the day is Sleep. Do you use it on the two goblin guards, or save it for a later fight where you might need it more? See, the choice matters.
I find it difficult to believe that someone involved in the design of a system as math intensive as 3E has a problem calculating a 5% bonus to earned experience but sees nothing wrong with stacking bonuses from many sources and recalculating these multiple times during combat as buffs are applied and dispelled.
Indeed. XP bonuses are applied out-of-play; nobody is waiting for you to get the math done. But ever-changing combat bonuses slow every round of combat.