Driving a standard diesel to a mapped one is a huge difference.
As standard TDIs have a very flat delivery, they peak and then sort of fade away very quickly. With a remap you will double your powerband width, from around 1800-2800 in a normal car to about 2000-4200 in a mapped one.
You also gain fuel economy because of the way the boost and fuelling is done, making it more efficient at part throttle openings, meaning you can expect around 6-8mpg increase on a steady cruise.
If you don't drive many miles you should not have really bought a diesel to be honest! That being said driving a GTI even around town the difference is huge, the worst I got from my 1.9 TDI was 450 a tank, the worst I've had from my GTI is 260 (so far...) so really the TDI is more economical.
But hey!