While fighting in Poland would create some tactical and operational benefits for Germans, it would also mean that Soviet Union gets to fight with their entire population and industrial capacity still intact. That cannot be a good thing for the Germans.
For more or less, the germans had to fight almost the entire "valuable" (poor choice of words, but..) population of the SU - after the attack, the SU made some great efforts to evacuate the "valuable" population (for example, when they moved the industry, they moved the workers too) and they blitz-constripted the able male population (sometimes i wonder, that not Zhukov saved Moscow, but the soviet buerocracy). At least, they seriusly tried.
While i did not seen nowhere - and likely, i will not see any in the near future, since not too many historians got into the archives of Moscow - any reliable statistics or records, how many military aged male and industrial worker got evacuated, it seems, that they were quite succesful. Same applies to the industrial capacity - of course, the loss of the Donetsk-Kharkov region or the (partial) blockade of Leningrad (and the evacuation itself) did not helped the overall equipment/supply situation of the SU, but together with the LL, they could overcame.
Question is, will they get the LL if they attack? I say, yes - after all, the Uk and US still want to defeat germany - and that meant more tanks/planes/guns for the RA. Question again: could that soviet material gain outweight the skill/manpower/supply loss? Could the germans in this situation (lets say, one month extensive fighting in Poland until the RA front elements get destroyed) couterattack and reach their operation limits (and during that destroying the second "wave" of soviet armies)? Well, i think, yes, a sucessful counterattack would be as disastrous for the soviets, as the first phase of the Barbarossa, and now they are one failed effort behind OTL - overall in a worse state.
