I think iAuto is P plus an automatically selected SCN. From what I read from the manual, P is automatic A and S, no SCN. If iEnhance is actually about automatic SCN selecting, then iAuto might be indeed just P + iEnhance.
I've done some brief comparison tests and iAuto does look very similar to P + iEnhance. However, sometimes I found an iAuto out very impressive, but I did not take a P + iEnhance at the same time. I have not seen a very impressive P + iEnhance out yet, which makes me feel that iAuto does more than that. Maybe I simply have not done enough P+iEnhance.