depends how you use it.
Induction kit are (in theory) suppose to increase the flow of air into the engine.
As a result the fuel has to increased to match the air/fuel mixture. So if you boot it it'll improve performance - but hurt your MPG
However if you tickle the accelerator, the increased air will improve conbustion so you'll need less fuel to maintain your speed - hence better MPG.
Depends how you behave.
Open inductions will kill it as hot air will be entering the engine, not not a high density. there less fuel will be injected to match the air/fuel ratio.
etc
