The ECU light can be solved with the sensor spacers -
EDIT : If thats what is causing it!
I think they can turn them off on a remap if you're having it done.
300 - maybe, maybe not maybe more you won't know until you put it on a dyno, also depends what you had before.
Fuel is not the only factor, each engine will be different out of the factory then there is wear and tear to take into account.