I didn't notice any mpg increase with my volant and mine came with a plastic lid. I have the powercore model (oval shaped ) and i have to check it at least once a week because it trys to slide off no matter how much i tighten the clamp.
I had the drop in filter. You can feel a difference in the mid high end torque, or atleast i think so. I havent dyno'd yet, but it does sound a lot louder when punching it. gas mileage is a mystery, it seems the same, i havent really measured, but i definately want to step on the gas more with it. :coolsmile