After a lot more research and digging, I've fixed the issue. For those of you wonder how to recreate the Silverlight TabControl default behavior of equally balancing tab amount per row, you have to create a custom ItemsPanelTemplate like this:
Add the reference
and then in your custom RadTabControl style add:
which will then force the RadTabControl to use a TabPanel rather than a TabWrapPanel as the ItemsPanelTemplate, which is what the default Silverlight control uses. The TabPanel will take care of all the nice wrapping and spacing of tabs on the rows.
Best of luck!