As per Generalissimo's request I rechecked the Polish POPs. According to the Russian 1844 census Congress Poland had a population of 4,769,000. However, it did not include all the areas and focused mostly on urban population while Poland was mostly a rural region.
Currently in POP mod Poland has 5.485.000 people (grouped in 108 POPs). This is my estimation based on the abovementioned data and seems reasonable. I will prepare some glitch in POPs distribution, but the overall number is rather correct.
The changes will include:
Less POPs in Siedlce (Siedlice in Vicky; currently 2.221.000)
More POPs in Warsaw (currently 1.391.000 while the area had approx. 1.700.000)
More POPs in Lublin (Spiff in Vicky; currently 255.000 while the area had approx. 1.000.000)
More POPs in Plock (203.000 vs. 530.000)
More POPs in Suwalki (real-life Augustów; 207.000 vs. 620.000 IRL)
More POPs in Radom and Kielce (386.000 altogether vs. 950.000 IRL)
More Jews (currently 7.9%, should be approx. 15% and rising rapidly)
More Ukrainians (currently 0.5%, should be approx. 5%)
Cheers
PS/ See
here for reference
Edit/ also note that due to Vicky bugs the non-national cultures are developing much slower than IRL. There's no chance Poland's population grew as fast as in history (almost 15.000.000 in the German census of 1915).