hmmm, your thought process is logical. but i know enough about the way ford computers run an engine to tell you that might not be the culprit. ford computers have a function called dashpot. what dashpot does is controls the amount of air allowed through the IAC valve under idle conditions. when you add power with your heads and overlapping cams, this function is off by a margin of error, be it adding too much air through the valve, or not adding enough. this is normally the cause of a rough idle on the focus. other than that it could be a MAF calibration problem, but im assuming yours has not been messed with. my guess is with the new heads and the poverlapping cams, the air is obviously getting through the compression chamber faster, therefore you most likely need to add more air through the IAC at idle. what year is your SHO? if its OBD-2 i can datalog it to tell you if it is what i think it is.