The easiest and quickest way to remove the rounding of the edges is to set the Shape property of the RootElement of the form to a RoundRectShape with Radius equal to 0. Here is an example which demonstrates the approach:
RibbonForm : Telerik.WinControls.UI.RadRibbonForm
We will consider removing the rounding by default for the Metro themes in a future release.
I hope this helps.
the Telerik team