In a bowl, whisk together the butter, olive oil, lime juice, chili powder, salt, and pepper.
Preheat the grill to medium-high. Brush a little of the butter mixture on each ear of corn. Grill the corn, turning the ears often so they cook evenly, until lightly charred, about 10 minutes, baste each ear 2- 3 times as turning.
When cool enough to touch, stand each ear of corn in a large bowl and, using a sharp knife, cut down along the cob to strip off the kernels.
Add the tomatoes, onion, cilantro, feta, olive oil and white balsamic vinegar, toss to coat evenly.