Pak43 =/= Flak 88
The Super Pershing would be much more OP than the flak gun or Sturmtiger. The limit to realism is what you could fit in the game and have work with out being to retardedly good.
Although the Sturmtiger is insanely rare as a unit in game, so I guess that's kind of fitting. (Although I adore it).
Pak 43 is literally just an 88 purpose built for AT. The only difference a flak 18 emplacement would have is the ability to shoot at aircraft.
Honestly if they manage to fit something as stupid as the Sturmtiger ingame, then the super pershing isnt too far fetched it did see some combat.