ValidationErrorHandler does not get invoked when validating in view mode and System.ComponentModel.INotifyDataErrorInfo interface is implemented.

New row in Bottom position is not visible in the child grid when the grid has no items

HeaderCheckBoxStyle of GridViewSelectColumn returns TextBlock as Header in Office2013 theme

Let your data column have a cell style with DataTrigger which sets the Tooltip depending on the value:

<telerik:GridViewDataColumn.CellStyle>
    <Style TargetType="telerik:GridViewCell">
        <Style.Triggers>
            <DataTrigger Binding="{Binding StadiumCapacity}" Value="42055">
                <Setter Property="ToolTip" Value="THIS IS TOO SMALL" />
                <Setter Property="Background" Value="Red" />
            </DataTrigger>
        </Style.Triggers>
    </Style>
</telerik:GridViewDataColumn.CellStyle>

Initially, when the grid loads - the tooltips are set correctly. But if you edit a cell and change its value to 42055 then the background is set to Red (as expected), but the tooltip does not update.

The issue is reproduced only with GroupRenderMode="Flat".

The reason for the performance issues is the size of the ItemsSource for the combo column and the internal lookup logic.

A new IsLightweightModeEnabled property of GridViewComboBoxColumn will be introduced with R2 2016. Its default value is False. 
When set to True, the new lightened lookup logic is triggered, thus the performance of the column is significantly increased.

RadGridView.ValidationType property will be available with R2 2016. 

With R2 2016 RadGridView will introduce ValidationType property that indicates what type of validation input should be processed(INotifyDataErrorInfo, IDataErrorInfo, DataAnnotations).

With R2 2016 GridViewComboBoxColumn will expose a property IsLightweightModeEnabled. Setting the property to True should resolve the problem.