My 140 is mapped and I usually get about 530'ish miles to a tank (mix of city, spirited and motorway driving) Worst I ever got was about 420 miles and the best was 630 but that was because of a few very long journeys and driving like a granny.
I regret not going for the 170 tdi and mapping it to 200. Only thing that put me off was the dpf. I was doing a lot of city driving at the time. Its how you drive it, put the boot down in the 140 and it'll drink diesel as well, drive it normally and it's a miser. I think the 170 might be a little bit heavier on diesel, but not significantly.
I love my 140, its a great car......but if I had a choice again it would be the 170. Would you be able to afford a GTD?