Just wrote an article on Embarcadero Community site you might be interested in:
community.embarcadero.com/index.php/article/news/15977-windows10withxe8
It covers installing RAD Studio XE8 on Windows 10 preview and building applications for the preview of the coming version of Windows. We are expecting the migration to be smooth, but also very interested in creating "first calss apps" (or universal apps) via the Centennial bridge.
We are working with Microsoft to offer Delphi and C++Builder the best support for Windows 10, whether they keep going with the best Windows library out there (VCL) or follow the multi-device route with FireMonkey. Stay tuned for more information.