That statement makes me unsure of the purpose of your question. Are you asking how the Sheaim can deal with Burning Sands, or are you saying that the conversion of Deserts to Burning Sands should be an improvement for the Sheaim?
Im saying it seems strange that the Sheiam suffer from a hightened AC. Since hell terrain spreads allmost instantly in their lands, and allmost never in good civilizations, then they basically suffer MORE from a high AC than good civs..