I would also be very sceptical of these figures. I had a 1.9 with a Spider tuning box and they never claimed more than about 185-190 maximum. The 2.0 JTDm isn't all that different from the 1.9 in reality - a slight capacity change, closer coupled DPF and a different map to release 170 bhp, so 210 bhp sounds like optimistic marketing speak.
Unless they changed the clutch when they moved to the 2.0 litre diesel, I would be wary of too many flying starts because the 1.9 clutch is known to be a bit soft and mine showed signs of slipping at less than 40,000 miles (less than half of those with the tuning box in place)
And yes, a tuning box is likely to screw up your fuel consumption computer and the only way to check it conclusively is an accurate brim to brim.