the negative side of the coil is triggered inside the box. That is what that transistor does. It is a switch. It closes and opens the negative side of the coil when the magnetic pick up in the distributor tells it to.

So ya, more voltage and current to the coil, the more power thru that transistor so it WILL get hotter. If the heat cant be disapated fast enough, it will blow or melt that potting material