Heat grill to high. Soak corn in cold water, turning occasionally, 10 minutes. Transfer to grill. Cover; cook, turning, until husks are charred, 10 minutes. Remove.

Using a kitchen towel, pull back husks; remove and discard corn silks. Return corn to grill (with husks pulled back and off heat). Cover; cook, turning, until kernels are charred in spots, 10 to 15 minutes.

Combine reduced-fat mayonnaise, lime juice, and chili powder; brush on cooked corn. Dust with finely grated feta cheese. Season with coarse salt; serve with lime wedges.