Justin, the way I achieved this was to use the ZoomChanged and ZoomChanging event. 

Code looks something like this. I did this to make the zoom on the chart instant.:

private void XamZoombar_ZoomChanged(object sender, ZoomChangedEventArgs e)
        {
            HandleXamZoombarZoom(e.NewRange);
        }
 
        private void XamZoombar_ZoomChanging(object sender, ZoomChangeEventArgs e)
        {
            HandleXamZoombarZoom(e.NewRange);
        }
 
 
        void XamDataChart_WindowRectChanged(object sender, Infragistics.RectChangedEventArgs e)
        {
            if (IsZoombarChanging) return;
            if (this.price_xamZoomBar == null) return;

            XamZoombar xamZoomBar = this.price_xamZoomBar;
            try
            {
                IsZoombarChanging = true;
                Range newZoombarRange = new Range
                {
                    Minimum = e.NewRect.X,
                    Maximum = e.NewRect.X + e.NewRect.Width
                };
                
                xamZoomBar.Range = newZoombarRange;                
            }
            finally
            {
                IsZoombarChanging = false;
            }
        }
 
        bool IsZoombarChanging = false;
 
        private void HandleXamZoombarZoom(Range zoombarRange)
        {
            if (this.price_XamDataChart == null) return;
            XamDataChart xamDataChart = this.price_XamDataChart;
 
 
            if (IsZoombarChanging) return;
            try
            {
                IsZoombarChanging = true;                
                xamDataChart.WindowRect = new Rect
                {
                    Y = 0,
                    Height = 1,
                    X = zoombarRange.Minimum,
                    Width = (zoombarRange.Maximum - zoombarRange.Minimum)
                };
            }
            finally
            {
                IsZoombarChanging = false;
            }
        }

Hi Justin,

You are correct the bound range value is not updated until the XamZoombar move has completed.  The values you are looking for are passed in the ZoomChanging event arguments.