Yeah, now that we're in the age of computer flow simulations and 'biggest number' car shoppers, you're not going to get more power out of something that simple or else the big manufacturers would have already done it. Especially on vehicles that make peak torque on the low end of the rev range.
They are really nice to have on some bikes though since airboxes tend to be a huge pain in the *** to work around in a tight frame.