With my tune my mpg as gone up from 24 average to 26.5 avg. i can tell my lead foot does affect my numbers sometimes i can avg. 21 mpg. I have some problems with my back O2 sensor as well because i put headers with a little cat that does not do much. Actually during my tune, the headers fried my front o2 sensor making my car not work properly. Then at the jag shop we switched the front with the back rerouting them so they are not as close to the headers. their is a o2 simulator you can purchase but i do not know if it will work for the r56 here is the web site O2 Simulator, E-commerce the would get rid of the CEL
The R56 apparently has fairly sophisticated O2 sensor software that a straight forward voltage divider does not work on. I have also noticed a drop in MPG's on my R56 since the tune. I also notice that my ECU is frequently limiting the boost to around 10PSI. My boost will spike to 17-18PSI for just a little bit but then settles in around 10PSI. I have asked Jan whether this is normal but have not gotten a response yet.
Well, on my R53 my post tune highway mpg went from about 28.5 to about 30.5. Not record breaking, but definitely good for this car with my mods. BTW, this was a re-tune for me as my first RMW tune was Oct 2008 in Plano. The re-tune was much much cheaper. I am still very happy with the tune for my car, the idle, the pull, responsiveness, etc. The R56 is relatively new and maybe somewhat of a work in progress. A re-tune might be in order for you guys to optimize that tune. I may be mistaken, but I think Jan indicated that he had a few more R56 tricks up his sleeve that was not fully tested yet, and so not yet released for customer cars. Some of these tricks might be the tweak you R56 guys need.