After carefully reading your problem I got up with the idea that your suggestion 3) is the best and here is how you can achieve it:
Use Custom Numeric Format Strings
to represent the values in MB and remain the Values of the Bars unchanged:
radChart1.DefaultView.ChartArea.AxisY.DefaultLabelFormat = "0,,";
The "," will divide the Axis Value by 1000 so add 2 "," to gain the MB representation.
the Telerik team
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