Whether Nitrous has any affect on mileage doesn;t matter, as you won;t be onteh nitrous long enough to get 'mileage'.

10 or 15 seconds at a time. You may get a minute of runtime on a a bottle? (not sure, but it ain't much). Keep teh gas on more than 15 seconds, your either gonna blow up the motor or or wet yourself from the acceleration.

Youcan get nitrous systems to add 100 HP up to 3 or 400 HP.

Speed costs money. How fast can you afford to go?