Problem is, the Allies would have a much easier job of delivering chemical weapons into Germany, especially by 1943 when the American and British bombers practically ruled the skies over Germany. The Germans knew that if they used their chemical weapons they could expect a disproportionate response from the Allies.
I guess this is why chemical weapons weren't used in WW2 - both sides were unwilling to risk the terror and destruction of chemical warfare...
EDIT: In the short term, the use of chemical weapons at Kursk could have helped the Germans in their attempted breakthrough, but I personally don't think that the Germans had the means to carry on an offensive war by 1943. The most they could hope for would be a much longer (and more bloody given the guaranteed mutual use of chemical weapons) war of attrition in the East. If the Americans and British started dropping gas on German cities, however, I don't expect Germany would have lasted very long.