Here is a IS2 taken below 50% by 2 passes, the schwer does a very minor contribution, it looks around 30% health after these two passes, I highly doubt the schwer contributed 20%..
https://clips.twitch.tv/RefinedStupidAsteriskTriHard
This ability almost always deals maximum damage within the first 10 seconds and rarely does limited damage. It probably is OP.

The math does not add up. Something is funky with this ability.
TBH, it looks around 35/40% based on the HP bar and the location of the vet1 star.
The IS2 already lost 5%-7.5% just before killing the Stuka. Right before the planes comes, you can see it getting rear shots (check the crits) from the Schwer. I don't think that the P2 did anything at all.
IIRC the Flak deals 40dmg per shot, which should be around 4% of the IS2 HP per pen. So we can say it lost a bit more than 10% HP.
His estimation might be a little under, but is more than likely that the IS2 received around the 500dmg mark.
I think it's a matter of just running several test to see how much dmg it's doing on avg, on static/move vehicles and see if something is wonky.