How big of a kingdom? Originally Duchy of Warsaw was made from Polish territorries occupied by Prussia, in 1809 augmented with part of Austria-occupied lands. Do we make KoP bigger, and if so, how bigger and with what lands? To make Poland "a full pledged kingdom" IMO it would have taken of all Prussia most of Austria occupied lands, with Warsaw, Cracow, Poznań, Lublin and Zamośc, probably also with Gdańsk (Danzig) and perhaps even Lwów (Lvov, Lemberg).
Personally I believe that would have been a good solution for France - Napoleon would have had a relatively strong and loyal ally protecting his empire from the east. Any attempt of Austria and/or Russia (not to mention Prussia) to attack him would have required dealing with Poles first. That wouldn't have been so easy (as it was demonstrated in 1809 IOTL, when tiny DoW quite successfully fought against Austrian army) and it wouldn have gien Napoleon precious time to react. Poland would have been a useful basis for any operation in Central-eastern Europe and French watchdog in that part of the continent. Poles would have remained loyal to Napoleon (as they mostly did IOTL) not because they would have ben grateful, but because the fall of Napoleon would have been equal with fall of Poland, since all 3 powers would try to regain thei Polish lands.
The only problem I see is if Napoled would have been able to do it. In 1807 French Army, while victorious, was exhausted; Austrians were preparing for a rematch and Russians still were not out of the picture. Napoleon negotiated with Alexander not only because he wanted to be recognized as an emperor by the European monarchs, but because his own soldiers were already tired and had enough.
What could have changed that? Prussians, Austrians and Russians being beaten even more soundly by the French. Perhaps if the French messenger wasn't captured by the Russians before the battle of Eylau, resulting with the battle being an absolute disaster for Russian-Prussian forces and wth relatively small French losses? Thaen quick victory overs the Austrians.