Say for some reason, Prussia retained her 1795 borders onwards. With more Eastern land, what could her potential be? Would she be as strong as OTL or weaker since she doesn't own the Rhineland?
Significantly weaker, especially once the Industrial Revolution kicks in and Prussia doesn't have the Ruhr and Saar coal fields, or the Siegen ore fields. But even before - well, 'New East Prussia' was barely settled swamp and forest land, and even 'South Prussia' was certainly not as economically developed as the Rhineland. The Russo-Prussian plan at the Congress of Vienna to let Prussia have Saxony in exchange for Poland (including Posen) would have been excellent for Prussia; getting the Rhineland for Poland (while also keeping Posen) was still a good trade, and would prove to be a miracously wonderful exchange once the importance of coal became clear - but just keeping Poland would have been meh. Prussia would still have the Upper Silesian coalfields, but would probably firmly stay the weakest of the great powers. Without growing into Germany, staying at its eastern edge, while keeping much more land outside the German Confederation (or surviving HRE, if the borders are 1795), it might also not take part in German unification.