I may be having a bad day.
ATT had all the American aces we then knew off in it who served in the RNAS/RFC and RAF. We might, I can't remember have found one or two more in the updates. The updates were needed because further records were released in the PRO and some kind and knowledgeable people helped with info. Bill Bailey put the USAS and French Air Service aces in it. That covers all USA aces in WWI. My guess is by late 1918, some 15 per cent of the RAF was American, the published Schiffer book listing all American aviators did not even get close. Hopefully they too can do an update.
Sometimes I wish people would actually know something before they spout a load of drivel on this otherwise excellent forum