I've never really looked into this, but is there a point where it's outright bad to do this even if you just use the leveled up piece as xp for another? I'll usually get anything that has a good main stat + crit to +9 then dump it into something else if everything is rolling bad. I'm not sure how much you miss out on doing it this way.
Technically no, there's always a chance to hit your desired stat.
And even at 15, you get some (all? I'm not 100% sure) of the excess XP back as more mats.
But with the vast increase in weapon XP required at higher levels, I generally don't go above 6 if I'm not getting either of my 2 desired relic substats to proc - maaaybe 9 if I've got some mats/castoffs to spare.
But it also depends on A) Whether or not I have ANY suitable gear for the set in question (i.e. if it's the first piece, I bear with going to 15 even if it's bad substats), and/or whether it has 2 or 3 desired substats
(And C)Whether or not the gear has only 3 substats to start with, or all 4).
....That being said, the number of times I've had all 4 levels ups go into the one solo flat HP or DEF stat is - or at least seems like - far higher than I would like/it should be.
Folding one armor piece into another does lose you some XP compared to just feeding materials directly, yes, though I'm not sure on the amount/%.
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