I have removed the water-cooling from my turbo, and my auxiliary radiator is fed from the head, so is a "true" supplementary radiator .

Your cooling system is already compromised by having to deal with the turbo . However, given the temps you're seeing, I think you can safely say you have cracked it . However, I attribute that more to the design of your front bumper. Unless you're prepared to do something similar to a Cossie, then the Pro-alloy item is the way to go .