Most of us who have a box have the crdt2 as the 3 is quite new (an upgrade is available).
I drove for 10k miles on 3+1 and 1+1, and have done another 8k since on 2+2 which is great.
DTUK recommend sticking to +1 to reduce risk of limp mode, I have increased it just because car has been fine at +1. I did start adding 50ml of millers to every tank though, as I was getting a fuelling issue at 1900rpm when flooring it in 5th (slight judder). Has been perfect since doing that.
Power delivery is very smooth and the crdt3 is supposed to improve that further.
I still get 50+ on motorway, and can still get high 50's when keeping to 70/75 so any reduction is barely noticeable.
I recommend a pedalbox too, makes the extra power instantly accessible!
Of course, your mpg could drop a bit if you are having more fun due to the box
Poll