How effective was tactical airpower against tanks on the Eastern front?

That effected StuG and StuH production and was critical to German doctrine, which was why Daimler was switched to produce StuG-IV and why it became so important to get Alkett up and running again. You can see the effect by the monthly figures - production peak in 1943 of 406 was in October, dropping to 299 in November, and 234 in December, then rising to a new peak of 551 in July 1944 before trailing off to 508 in January 1945 (no reliable figures after that). The CBO never concentrated on the StuG and StuH production, except for the Alkett attacks. Furthermore, after January 1944, BMM and Skoda were switched to producing the JgPz-38 (t) from the Marder, which inflated the production figures of armored vehicles as did the conversion of some other plants to vehicles such as the Jagdpanzer-IV, StuPz, Jagdpanther, and Jagdtiger.

You need to read and understand what was written. The major attacks in 1944 on the Panzer assembly plants began in August. In July Panzer production reached its wartime monthly peak of 840. It decreased in successive months to: 777, 622, 566, 566, 666, and 447. After heavy bombardments. Add to that 177 JgPz-IV, StuPz, Jagdpanther, and Jagdtiger types in July, increasing by January to 317. So 1,017 to 764.

It was only production of the marginally effective JgPz-38 (t) that resulted in an "increase".

 

Actually, they were "immediately successful". The problem is that immediate success was often too easy to recover from. What did the damage at Henschel was 15 days of concentrated attacks - five major ones - that took three months to recover from. The problem was there were some ten major assembly plants, each requiring the same amount of effort...and reasonably clear weather for a really effective attack. It is possible such an attack beginning in say May and continuing through September would have a sustained effect...except there was also other targets.