Since I've been R-Tech stage 1 I have noticed on average I am getting 2-3 mpg more on the standard driving which is a mix of motorways and town driving BUT and its a big BUT it is just as easy, if not easier (purely because its now so much fun) to drop well below what I was getting pre remap!
I say get it mapped by R-Tech, save yourself about £200 from most other tuners and put that towards a few months fuel, you wont be disappointed!!