pleas

+hits target applies poison (simalary or same damage as fire that slowly kills player but never goes away excluding health pack with an effect like jarate making the screen green except off yellow)
+50% reload speed
-80% ammo
-30% projectile speed

so poison and itr last forever? but if you take a health pck it goes away?

Can the spy also have a slightly faster weapon switch with this on?

i don't see why not :)

on hit: 35% slower move speed and marked for death both effects lasting 5 seconds.
20% faster reload speed

negatives: -35% damage and fires projectile instead of being hitscan and therefore has a travel time.
-80% clipsize (1 dart)

Also, I have a suggestion for a better name. Why not call it the sleeper cell? that is a real term and, as far as I know, sleeper agent isn't.