With Poles fully prepared German losses go higher and their progress is slower. Poles migth have avoided a lot of confusion caused by starting mobilization, calling it off and starting it again. With all Polish units positioned when they were supposed to be there would be less confusion during the war itself, when some units after being mobilized couldn't reach their destination and find their army/group commander. Army "Prusy", Polish main strategic reserve, would have been ready and waiting against main German thrust. They wouldn't have defeated it, but they might have been able to stop them for same time and inflict some losses on them. Operational Group "Wyszków" might have been able to hold position on the Narew rover for some time. Etc., etc.
However, Polish defensive plan was flawed, Poland's strategical position was terrible and German Army was superior to Polish Army in every aspect except courage. Poles had no chance even to fight Germany to a standstill, and they knew it. Polish strategy was to hold as long as possible to give the Allies (France and Britain) time to organize their own offensive from the west, as it had been promised.
The French (and the British) weren't to keen on attacking, and believed, correctly or not, they were unable to give Poland any serious help. They launched the Saar offensive, which was half-hearted at best and considered it done.
I think Stalin might have attacked, only he might have waited a bit longer. IMO he would have attacked pretty much at the same moment he did IOTL - with Polish Army practically already defeated and Allies being passive in the west. It might be 24th or 27th September instead of 17th, but he would have attacked.