By definition the Panzer II Luchs is a reconnaissance tank, and yet in the game it functions as a light tank. It's almost as if real life combat roles and weapon/vehicle charactaristics are sometimes bent into different roles in the game to serve gameplay.
An extra shell per barrage is not weak.
As for vet 5 the +20% damage increase is pretty huge.
Half the reduced scatter perhaps, but its base scatter values are much better.
Better scatter for all fire modes is also much more valuable mid-late game (when you largely rely on autofire) than just a barrage scatter reduction other mortars get.
Really touch and go for either description. A Panzer II was a light tank by definition. On the other hand, an Leig was vastly heavier than a mortar (easily a difference of 500lbs or more). There's no way three people could take one apart and carry it around with their own gear, along with a cache of ammo.
That being said, maybe it'll feel more like a howitzer if the vet changes go through. It's pretty tough to get one past vet two or three, as of right now.