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Pretty crazy! The junkies do get really strong by then, with multiple thousands for pain and lust meters - and they hit like a truck, too. It was basically impossible for me to progress normally, by that point! 

I was having to resort to quicksave-scumming my way through. Walking into a junkie encounter generates a different junkie or boss each time, at encounter time. So, if you quicksave, walk into the encounter, and the junkie is impossible: reload, and walk into it again - the junkie will be completely different. c: You can do this until you find one you can fight. For example, if you've been maxxing Sexiness and sex perks for Lust attack effectiveness, keep re-rolling the junkie until their Lust bar looks more favorable. (Some junkies are balanced, while others have really high HP coupled with low lust - and, vice-versa.)

Another way you could technically progress is just submitting to junkies you can't beat and letting them all have their way with you. ;)
Unfortunately, there's two issues with this: you won't gain experience this way, quickly becoming impossibly out-scaled after a point (earlier than usual); and, most junkies will just opt to slap a bunch of restraints on you and choke you out, completely. (I'm not sure if it's broken for other NPCs, too, but) junkies will ignore your settings for encounter restrictions that could normally be set to disallow NPCs from choking you, etc.  
To get around this, again, you'd have to save-scum: quicksave before the encounter, walk into it, and submit - and, if they proceed to choke you out, reload. Re-roll until the junkie only fucks you silly and then just lets you go. Both bosses and regular junkies have a chance of letting you go, afterward.
Theoretically, you could progress this way indefinitely, since submitting like this clears that encounter square on the map! Although, sometimes reloading so often causes certain markers on the map to not show up again - or to get stuck (visual-only, there won't be an encounter there). When you progress to the next level, it will fix itself. c:

That's about all I can think of to share, so far! After unlocking all the drugs and progressing Kidlat's and Quinn's stories, it doesn't seem there's too much more use of going back into the drug den. I'm looking forward to folks adding mods that make more use of this module, in the future!