Oh dear - nothing is ever simple is it! Whilst the different sized feet may not be an issue on its own, the combination of that with the lameness/soreness problem would worry meDoes the vet think she is lame/sore in more than one foot? How overdue for shoeing is she? If she has gone a very long time between shoeings, then some soreness maybe excusable. Exactly what level of competing do you want to do with her?