The upgrades in Tanktastic come in three main areas.
There will be an ability to modify the Turret/Mk of the tank in a future update.
This changes the physical properties of the tank. Armour, Engine, Speed, Turret Rotation etc.
This allows you to penetrate different types of armor and gives the ability to inflict greater damage.
This has not yet been implemented into the game. We do have some information regarding possibilities.
These can make you less of a visible target on certain maps and can be used for aesthetic purposes.