If installed correctly, the action bar at the bottom in the main game screen should expand to screen width. However, all menus (saving / loading, inventory, character record) have a fixed resolution (1024x768 I think) and do not expand. They're drawn centered on the screen, so if that's what you mean by 'destroys the UI', then yes, that's normal. So if you set it up correctly, you should be fine with GemRB 0.8.5 and the latest widescreen mod.