This is an interesting question Leo. Considering that it seems to have taken at least a full year for the effect of US entry to have a significant effect, the Germans would have had to endure the same waiting period the Brits and French did.
Shortages of German manpowerand resources would have been overcome in such a scenario for sure, but America was not yet the military giant it would become in later years. Obviously American entry into the war on the other side would have changed things significantly. Whether or not it would have altered the final outcome, I don't know. I wonder if the US-Canadian border would have been the location of a third front if such a scenario had played out. Is it possible that Bishop and Luke might have dueled over Detroit rather than Douai ?
Didnt Churchill suggest that US entry interfered with a likely settlement in 1917?