Yeah I confirm I got all blueprint, the sleeping darts are kept to 10 no matter what. You can get a bone charm that boost the ammo you find though. This is probably so that messing up in a stealth walkthrough has more consequences. And you don't just advance carelessly and can't just whip out the tranq gun everytime you are spoted.
About that room with weepers.I guess, tranq darts, stop time, possession should work.
Well, that sucks. I'm currently playing as stealthy as possible and have no use for a lot of the upgrades that are available at this point(just after the Overseer mission).
What are some that I should look out for?
I ended up picking up the sneak upgrade (blueprint, can't remember which level), mask zoom upgrades (blueprint, bought from Griff on the first level after you free him), and crossbow upgrades. All the accuracy/range/etc crossbow upgrades work for all bolt types, so they help with the sleep bolts as well.
I also picked up the increased bolt capacity, which ended up being useful during a later level when you're going through the sewers and you keep running into those acid-spewing buttholes (whose name escapes me at the moment).
The sneak upgrade is mighty. While it doesn't stop dudes from seeing you if you are standing, you make no noise while running. So you can run without being afraid of alerting guards. Really really useful in insides and crammed places with different levels. Anyways, you can buy all upgrades before the end game.