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.

RadGridView's FooterRow is still part of the visual tree, even though its Visibility is set Collapsed. 

Cell gets out of the edit mode when new items are added in the collection. The same scenario works correctly if alternation is applied by RowStyleSelector. 

The problem should be resolved with LIB version 2016.1.404.

If I change Column.IsVisibility to FALSE, the column group headers are disappear and not restored.

The problem should be resolved with LIB version 2016.1.404.

The problem should be resolved with LIB version 2016.1.404.

The problem should be resolved with lib version 2016.1.125.

The new properties will be available with Q2 2016.