Hmmmm.... Seems a bit wierd. The Panther is penning 117mm at 2000m range, and around 145mm at 1000m, IIRC.
So 130mm doesn't seem like an unreasonable estimate for 1500m.
Unless you're just hitting at an off angle, or on the sections where the armor from the turret and mantlet overlap, you should be able to penetrate without any troubles.
Personally, I would aim at the track if you're just not penetrating; it will do three things to help you.
1) he will list to one side, and so will throw off his aim, since his sight isn't adjusted for the new angle of the gun barrel, and is unable to adjust in the correct direction.
2) It will imobilize him, so he will be unable to pursue should you wish to withdraw, or move to face you if you reposition.
3) If you reposition, he will be unable to use his mils-radian gunsight to aquire your new range, and so will have to use the guess-and-check method.