Heli cabin
The first upgrade increases the capacity of the helicopter to 40, with each successive upgrade increasing it by 5.
In a normal campaign, there's no need for more than one upgrade as you would reach your rescue quota by two pickups.
Llama Mod
When you get Llama bombs, you can initially drop two bombs. The two upgrade levels allow placing an additional llama bomb, up to a maximum of 4.
Artillery Mod
This upgrade adds an extra barrel to the artillery, allowing additional shots per upgrade. The upgrades allow for 2 shots per volley, followed by 4, 6, then 8.
Elephantbird
Each upgrade doubles the clip size and shells per barrage.
There's no need upgrade this, as the fast reload rate and attack power already make it trivial to eliminate zombies.
Merc Training
This upgrade increases the experience rate of mercenaires. The first upgrade gives a 10% bonus, with additional upgrades increasing the bonus by 5%.
Catbird
Research into the Catbird cannin increases how quickly it gets charged. The first upgrade increases the rate by 20%, with higher upgrades increasing by 10% per level, up to a 40% increase.
Kraken Bomb
This upgrade increases the amount of damage explosions do against the mega zombies. Damage is increased by 50% per level, giving a total bonus of 250%.
Epinephrine
This upgrade increases the movement speed of scientists. The first upgrade is double speed, second is triple speed, and the third is quintuple speed.