There are various other bits of unlockable content in the game but none of it is special. Artwork and new character models or outfits add nothing to the experience. Even the additional game-play modes are uninspired. The Headshot Mode for example merely challenges you to play through the game killing everyone with a headshot.
The unlimited ammo and adrenaline along with the boost to the size of your health meter are potentially useful. However the game is not all that challenging to begin with so you may find you don’t really need them either.