On a side note; do people regard remapping their GTI as increasing the un-reliability?
I think you mean making it less reliable?? In an upside down way?
It will highlight any weakness in the spark plugs / coils, fuelling system, clutch etc. It won't increase the chance of a break down though. .
Re mpg: made no difference to mine. When R-Tech Nick mapped it, he left the light throttle map at lambda 1 so I still got 40+mpg on a run.
Used a lot more fuel when using the power though (foot to floor)
Ps: the mods will be onto you for that signature. It's MASSIVE