From what I've been reading, a big problem with having a kettle bigger than the element is that it triggers the thermocouple associated with the burner. It's there to sense the temp of the glass and the pan so the burner can turn on and off to maintain the selected temperature. If the pan is too big, it triggers the thermocouple and makes it think things are hotter than they are, so it never gets up to as high of a temp as it could.
If that's right, having a brewpot that's taller and narrower so it just fits the burner might allow you to boil larger volumes. So long as the glass top can take the weight.