Suppressors are useless IMO. The loud metal *clang* the slide does will still give away position.
Well, they should reduce muzzle flash, so during night it won't give your position SO well, that enemy could actually shoot at where he saw flash and kill.
My suggestion for silencer would be to implement some kind of sound visual feedback system. Right now I hear some footsteps and gunshots, but I've got no idea where they come from.
I have attached screenshots of my quick mock-up on how this could be changed.
Depending on how well your soldier heard some sound, he will have that much accurate feedback. He can hear many sounds at the same time, maybe that should be reflected. Can he tell difference between alien footsteps and civilian? What if sound originates "somewhere from above/below"?
My suggestions for other attachments:
Assault Rifle:40mm shotgun / 40mm GL / no attachment (lighter weapon, better to handle, more accurate?)
It should cost a little bit TU to bring up under-barrel sights
Sniper rifle:ACOG (4x zoom) / 8x scope / 12x scope / night vision scope (according to ufopedia, not all aliens show up on infrared) / infrared scope
less zoom = less TU cost and quicker accuracy drop with distance
specialized scopes should have static-predefined zoom?