Its easy enough for anybody to "fudge" a rolling road result if they know what they are doing, hell, even pumping up the tyres for a second run will give different results!
People should never get hung up on figures to an extent, the main use for a rolling road IMO is to judge your mods, travelling from different rolling roads and getting different figures means nothing, its better to pick one, and use that one over and over again, to see if your mods are having an effect.
The Subaru and Evo lads are generally numpties in this respect, preferring to go to "Place X" because they will get a better result over "Place Y" a completely ridicules practise, me personally, i would prefer to go to the one that's known for offering lower figures, as the chances are, you are probably getting real results and not fudged results where the rolling road operator held the handbrake on a little bit, or never fully backed off the throttle for the coast down to affect the calculations
Nothing wrong with remapping a car IMO, but a simple "fuel map optimisation" is never going to give what i would consider "good" results, unless the map currently in the car is not giving good AFR`s for the current mods/engine spec
Diesel remapping/tuning is a slighty different kettle of fish
i had an old vectra derv that used to run 1.35bar boost, that was pretty nippy, although i was using about 1 head gasket per year LOL
and killed a head, plus a turbo