I don't know, I have a pretty respectable opinion of the AEF. American divisions were BIG! In 1918 an American division had about 60% more manpower than a typical British or French division and twice that of a German division. With lot's of artillery and an unmatched logistics tail, I do not think that the US units are over rated although the tendency in British circles is to write them off as incompetent and poorly led. I suspect that there is a great deal of chauvinism in that perspective and too many British authors mock the US Army for taking longer than a year to learn what it took the BEF well over three years and half a million dead to discover about mechanized warfare.
For all of its flaws, I think that the AEF was the decisive factor in forcing Germany's capitulation in 1918.
As a Canadian I am hugely proud of the accomplishments of the CEF (whose strength is also exaggerated in WW1G) but I think that the game's designers really give the US Army its due.
If only it was easier to get them to the Front...
I wondered about that - the US should probably not start with three corps (even understrength). The US Army in 1917 was something like 25,000 men. The Marine Corps was less than 10,000 - they certainly should not be the most powerful corps in the game. If the US had one understrength corps and no RP to start, this would probably be about right. Then let them raise their army through builds. This would delay them an appropriate amount of time.
I don't think that there is any reason why this could not be done. You would need to edit the appropriate entries in the Nations.csv, Reinforcements.csv and Detachments.csv files in the scenario and the ProductionCosts.csv file in the DB folder. Maybe some others too, have not really looked at it. You would also have to script the AI so that when playing as the CP the US forces would build reasonably.
Not convinced that the results would prove worth the effort but you never know.
Maybe if I can get enough spare WW1G time I will try this in the 1916 Scenario and if it works, porting a Mod over to the Grand Campaign should be fairly straight forward.