This ability seems too strong to me. You get a massive damage boost against targets the axis relies very heavily on and it synergizes way too well with the P-47 and B4. Using this ability it seems way too easy to just coordinate a few button clicks amongst your team and take out a KT or Jagdtiger.
I suggest changing it from a received damage boost to armor reduction. I'd also shorten the duration or at least make it end when the plane gets shot down.
It's not a massive boost, it is 30% more damage. Only really useful on heavy tanks that now might take 1 less hit to kill.
Synergy with other abilities is seen quite often in this game and mark vehicle does not stand out above them in terms of power. OKW fuel feeding KT rush, flares+stuka strike, TWP+stuka close air support, button+bomb strike etc. The list goes on.
If a vet 3 B4 is giving you troubles, it just means you have failed to coördinate with your teammates. B4 is one of the very few tools allies have against a JT. Have a teammate with recon and an off-map and you can counter it with 2 clicks. It's called covering each others weaknesses.