I don't have data for old spots, but at ninja castle, the spawn is like I said, insane, and for lvl 50, the mob gives 20% more xp than book ghosts and you can go there 5 lvls earlier. The lvl 60 mob gives 13% more xp than sage cats and it's 6 lvls earlier. The lvl 80 mob gives 4% less than WS, but literally all you need to do is TC left/right like at teddies.
That's the problem with those spots. I found them early on, and I know they gave better exp than anything else the guides mentioned, but it's really difficult finding a safe spot for your leecher.
Did 30 min solo leech @ https://bbb.hidden-street.net/map/mini-map/zipangu-castle-corridor-4 Started off with 90% Lvl 55 ended up with Lvl 56 58.25% So around 135ish % per hour leech. Just casually tele left and right spamming gen not even TC-ing, however in term of loot unless you willing to spend time climbing up the rope to loot top lane, you won't gain any profit in term of npc-ing eqs