Hey!
I was thinking about this the other day - this is a bit of a janky solution but it should work...
superTextMesh.info[superTextMesh.latestNumber].bottomRightVert should give you the equivalent of superTextMesh.bottomRightBounds, but for whatever the current letter being read out is. So if you keep track of this value and use Mathf.Max like this:
Vector3 furthestPoint = Vector3.zero;
//as mesh reads...
Vector3 point = superTextMesh.info[superTextMesh.latestNumber].bottomRightVert;
furthestPoint.x = Mathf.Max(furthestPoint.x, point.x);
furthestPoint.y = Mathf.Min(furthestPoint.y, point.y);
I'll try to add a variable for keeping track of this automatically in the next update!
-Kai