Just got a chance to try your suggestion... as a simple test I implemented the following:

public class TestCreationFilter : Infragistics.Win.IUIElementCreationFilter
{

  public void AfterCreateChildElements(UIElement parent)
  {
   Console.WriteLine(parent.GetType().ToString());
   if ((parent is UltraExplorerBarGroupHeaderUIElement) && ((parent as UltraExplorerBarGroupHeaderUIElement).Rect.Height > 20))
   {
      Rectangle rect = parent.Rect;
      rect.Height = 20;
      parent.Rect = rect;
    }
  }

  public bool BeforeCreateChildElements(UIElement parent)
  {
    return false;
  }
}

This does work, but, this causes the explorer bar to take a VERY long time each time it redraws itself... if this is not easily achieved its not a big deal to me, I just thought I would ask.

Hello Pietro,

I wanted to know if you were able to solve your issue based on these suggestions or you still need help. Please let me know.