Jammy19

I guys, I've got a 200tdi, it has big off road wheels & rubber, it's pretty smoky, in its checkerd past it has had some power screwed into it, from a place in Mansfield apparently, you can tell it has abit of grunt but has no real power, especially above 30 ish, I did a 50 mile run yesterday and got about 17mpg, I feel like theres alot of fuel going in and not alot of power coming out, should I be expecting this level of thirstyness or can I get abit more bang for my buck?