It is certainly possible, but the devil is in the details. How big is reunited Poland? Do the Silesian Piasts unite all other Polish lands? Władysław the Elbow-High (Łokietek) managed to unite only Greater Poland, Lesser Poland, Kuyavia and Pomerelia...and then he lost Pomerelia.
Anyway Poland would be much more west-oriented culturally, IMHO; Polish kings still would be interested in taking Masovia from under Lithuanian influence (BTW, how is Lithuania christanized?) and that mind end with Polish- Teutonic alliance against Lithuania - if there is no earlier conflict between Poland and the Order. Such an alliance might lead to Poland conquering Masovia and some part of Red Ruthenia (possible competition with Hungary for the area), while the Order with Polish support conquers Samogitia. Anyway, it might end rather badly for Lithuania. Polish-Lithuanian alliance had many problems with defeating the Order IOTL - and now Poles and the Order fight together.
After that - who knows?