But, that's my point: if the Aliens have a quick-acting anaesthetic, then why not use it before the XVI lance as a general-purpose weapon? If you need to inject 100ml (lets say) of XVI into a victim to infect them, then you can't inject it too fast, or else their body will not be able to absorb it!
Maybe the XVI lance takes a ton of TUs to use it, so that an Alien using it has to basically spend the next round standing over their victim, sort of like the Medikit.
This leads me to: Alien psi/tech/chem stun weapons. Stun the target, and the XVI lance becomes much more easy to use! "I felt a malevolent presence in the room, but was unable to move or even scream. Then they started injecting me with horrible alien tools... It was awful..."
You know what I mean?