If the IMF numbers are to be believed...
https://en.wikipedia.org/wiki/List_of_countries_by_past_and_projected_GDP_(PPP)_per_capita
1980 Communist Poland GDP per capita: 4753
1980 Germany GDP per capita: 11273
So the ratio Germany to Poland during the height of Communism is (3/1) worse than today (1.7/1) and in 1938 (3/1) as the map on the last page showed. There's definitely post comunist development but the EU subsidies and extra trust in the Polish economy due to being inside the EU should not be ignored. Without WW2 and the Cold War there is no EU.
Without WWII and the Cold War we could expect more contacts, trade and movement between Germany and Poland, and between Poland and the rest of the world. Most likely there would be economic convergence, Poland slowly or more quickly catching up to Germany, depending on its policies. Compare to an example up north: in 1917, the GDP per capita was only half of that in Finland, a recently independent more eastern state, than in Sweden, a traditionally independent regional economic powerhouse. All through the 20th century Finland played catch-up (even if suffering disproportionately more due to WWIi) and by the 1980 came very close to Sweden. Germany and Poland would share a similar dynamic - especially if we remember that avoiding the massive destruction of WWII the Polish economy would be a lot stronger going into the 50s than IOTL.