You lose xp at the same rate you gain. As you said, if you are level 70 playing a level 5 shroud and getting xp at a level 5 rate, you also only lose xp at a level 5 rate. I imagine less than .00000001%. I am working on a Fairy shroud and I often take a level 10 version and do suicide xp runs in The Warrens. Its good xp and I die alot but it is fun. My lv 70 main does not notice any of it, the xp gains or deaths.
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After 16 years, I'm not listing every friggin character.