I don't think it's a good idea, a cacthing weapon with 100% tu cost would be mostly useless for the catching purpose regardless of its damage rating. I'd rather use it as a portable anti-tank/ufo breaching cannon (ie defensive use), it'd excel at this task
If you want a potent sleep toxin that's pretty much ineffective against armored targets, make it fire a lot of AutoShots with very good accuracy, but Power not exceeding around 25 - that's my suggestion. And 100% TU cost disqualifies it as a weapon which can be used to actively hunt - you can only put to sleep enemies who are storming your current position. You can't even snipe since sniper sometimes has to turn 45 degrees left or right at the very least.
Oh yeah and "infantry" thing. Mutons are almost as hard to stun as Cyberdiscs or Reapers, while for everyone else Stun around 70-80 is already an overkill.