FWIW, under the "1 max HP restored costs 100 XP" rule, I think I've only seen one PC come out of a fight with negative XP earned in that fight, and that was partly because he was letting the Vampire Spawn munch on him fairly freely due to his trollish regeneration. (Long story there involving ancient xixchil bioscience.) Remember that a Vampire Spawn is worth 1800 XP, and only drains 7 HP of max HP per bite attack, so it takes something like six rounds of biting (depending on AC) to drain 18 of max HP.
Obviously a wraith would be much worse. (Potentially 21 max HP drained per hit, instead of 7, although there is a Con save to avoid.) Chasme is even worse.
Anyway, I agree with the point that XP drain should not be so large as to kill multiple sessions' worth of progression. It is better if only (approximately) the current session's worth of XP is at risk. Maybe 2x that at most, but definitely not 8x or 10x.