You just take it off and then use VCDS if you have one to clear any fault codes. Mine went through with no issues. The VCDS does pick up a fuel sensor error when removed so would be very obvious to a technician that you have used a tuning box of some kind.
Which setting would you recommend for best performance, without being too aggressive?
Also what sort of improvement in MPG would you get on this setting (when you are driving sensibly of course)?
I had it on 2+1 initially but when I spoke to the guys at DTUK they said that 2 can cause engine errors which if you can't clear them is not what you want on your car. I never got an error myself however I moved it to 1+1. They recommend 1+2. But I didn't really experiment with it to be honest. I sold my GTD so I can't experiment anymore and I decided not to fit the box to my new R as it's much more complicated on the petrol engines than diesel.