I have also done my own very unscientific testing of air filters and housings in my 5 years of smart roadster ownership and agree with Breathless, there is no need to change the whole induction system, just the air filter, and for your bucks it is the best mod you can do on your smart, and one of the cheapest as well.
If you don't do anything else to your car, bin the original filter and get a sports filter, you get an increase in performance, a great sucky noise on acceleration, and quicker and smoother gear changes.
Do it, do it, do it, (sorry just watched Starsky and Hutch with Ben Stiller and Owen Wilson)

Ellis