Well, it would be a nice wank, but let's not go into ASB territory.
Problem with PLC is that eastern parts of the Commonwealth were to put it simply, empty - when compared to Western Europe. There is a reason why they were called "Wild Fields".
As for the Western border - people in the current Western Poland were almost always overwhelmingly German. Unless they are "Polonised" - which actually happened in many cases, but not in overwhelming numbers and not in areas where Germans were majority* - they will become a source of instability/discontent/irredentism/whatever come the age of Nationalism.
I have a hard time wrapping my head around idea that Poland takes the role of Russia through union with Muscovy. Such Commonwealth would be Poland in a name only - with nobility/ruling class adopting Polish language and mannerism (Golden Freedoms, sabres and moustache anyone?) with the rest of Russian/Rusyn/Ukrainian peasants speaking their own languages with their own culture and mostly Orthodox.
*Believe it or not, but many Poles have German-sounding names. At the uni I had Szmidt (Schmidt), Sznajder (Schneider) and Szwajgier (Schweiger). That is due to Medieval policy in Kingdom of Poland of importing German settlers to cultivate empty lands.