As of April 2016, AMD has finally released an updated Radeon Boot Camp driver for the 2015 Mac Book Pro Retina 15″ and similar i Mac Retina models.This updated driver fully supports Rainbow Six Siege and other new games.
Boot Camp reformats the flash drive as a Mac bootable install disk, and combines Windows 10 with install scripts to load hardware drivers for the targeted Mac computer.
The important Apple drivers which Windows invariably doesn't have updates for are Facetime camera, Apple keyboard/trackpad/mouse, and the IR receiver.
Beyond these you can update any drivers that MS has to offer through Windows Update, and I've had no issues with downloading video drivers directly from n Vidia.
Additionally, if you read the comments there are now a couple of sites that claim they have pre-packaged modified drivers available.
I’ve not yet tested them, so proceed at your own risk.