I have came across this useful and informative article by King Arthus Flour HOW TO REDUCE SUGAR IN MUFFINS. The experiment by the writer shows that reducing sugar to 50 percent in a muffins recipe, with sugar amount more than 50%, will yield muffins with sufficient flavour, moist and tender texture. But when the sugar level is already 50% or lower, reducing sugar will yield bland muffins with dry and tough texture.
Taking an example of King Arthur Flour Basic Muffins, the sugar level in recipe: 100g sugar ÷ 240g flour = 42 percent. The sugar level is already below 50%, reducing the amount will make the muffins drier and uneven.
King Arthur Flour Chocolate Breakfast Muffins recipe has 1 1/4 cups brown sugar. To maintain 50% sugar level, reduce the amount by about 2/3 to half cup sugar.