Yeah bud, XT-1 is pretty potent stuff, it has a plenty of anti-corrosive and biocide. The kill coil isn't just redundant, its also an extra type of metal to potential react with the 2 or 3 other metals in your loop, I would avoid it. If you've been using the rad already, well as Lutfij said, some of that gunk probably came loose already, you'll find it in the fins of your CPU and GPU block. It's not too late though, try one of Mayhem's Blitz kits, it's a two step flush that can clean even the dirtiest of Alphacool's radiators.