Doesn't Stuart have the same ability that allows this tank to stun then kill the vehicle by itself? And, like in Stug's case, a more powerful vehicle than itself: Puma. Is that OP?
Unfortunatly, you forget to take several things into consideration. The stuart ability requires the tank in question to be slugging it out within its own range.
The stuart has a range of 40 I believe, which puts it in the range of the unit it is shooting at. The Stuarts ability also does not increase pen, which means that the stuart can not frontaly engage heavier vehicles. This leaves the stuart with light vehicles to counter.
The stuart deals 80 dmg per shot, with 3 shots (240dmg). The stun is applied on pen, but stuart stun lasts for a lot shorter of a time frame with each shot than other stuns. This means that stuart stun will heavily damage a flack ht (which has counterplay in terms of smoke), and half kill the puma (assuming all rounds hit, which they often don't ). The puma is a scout unit, which gives it the ability to see the stuart coming, and thus not only evade it, but also deal heavy damage to it from range (Puma requires 3 hits to kill stuart). The puma can also smoke to evade.
I don't really see where you are going with this. Stuart and puma are both 70 fu AT platforms. The puma has the advantage at all times. Even if the puma is caught and stunned, the stuart still has to seal the deal with 2 extra shots. If you are caught by a stuart, you saw him coming, got a shot on him, didn't move, and thus have Noone to blame but yourself. TWP on the other hand comes from long range AT platforms that can fire from offscreen. TWP also increases pen, and deals 2-4x the damage.
And finally, even if the stuart ability was OP, that would not be a reason to consider TWP balanced. One OP ability does not excuse another. Now, do you have any actual arguments as to why TWP is balanced?