Is air-gap intake too much for a 318?

I would just use it. Your mechanic is partially right, you won't likely see all the benefits that the air-gap offers over the performer. On the other hand, I doubt you'll get worse results with the air gap then you would with the performer.

If you were going to buy an intake, I'd say save your money and go with the performer if you're planning on staying stock, but since you already have the air-gap I'd just use it and have it in case you upgrade your other components later.