Improved mpg by re-tuning

As a relative newcomer to motor caravanning I have been told there is a company called Quantum Tuning that offers re-tunes nationally to improve mpg and torque. I wonder if any members have heard of/tried such an improvement and what their feedback is? For example does it affect vehicle warranty? I have a new'ish Fiat Ducato based van and would love to get more mpg than my current 32-35. Their website certainly looks impressive.

TedK