If you're worried about fuel economy, adding more power won't help you!
Nitrous will also reduce your fuel economy as you are injecting more fuel to burn, the nitrous just promotes the burning. On the other hand though, nitrous is only used at full throttle when you actually want the extra power. Rest of the time you just drive with your standard engine.
Small amounts of nitrous can have big benefits on forced induction cars by further cooling the inlet charge.
If there's any questions I can help with, let me know. I'm an agent for the Wizards of Nos.