1. How sure are you of the aircraft weights?
2. What is the history of the annual stall speed checks (if these are CAA aircraft)
3. Have you been applying mods to just the one aircraft, or to both. Are you modding the faster or the slower staller, if just the one.
Im a bit suspicious of your statement that moving the stall strips is doing nothing. Does it USUALLY on this type. If so, then it sounds very much like something else is trigerring a stall earlier than the range of stall strip positions, and that LE contour issue you mention is a likely culprit Id have thought.
have you tried stall checks in other configs, such as clean. perhaps its a flap rigging issue?