Combine mayonnaise, sour cream, cheese, chili powder, garlic, and cilantro in a large bowl. Set aside.
When grill is hot, place corn directly over hot side of grill and cook, rotating occasionally, until cooked through and charred in spots on all sides, about 8 minutes total.
Transfer corn to bowl with cheese mixture and use a large spoon to evenly coat corn on all sides with mixture. Sprinkle with extra cheese and chili powder and serve immediately with lime wedges.