a FFIM won't make your car run like crap. It, however, will completely alter your power curve in a way you probably won't like. Expect to see a loss in tq below 3k rpm. In theory you would gain hp, but that would happen at or near redline. I could see changing out the intake manifold when the long runner design becomes a hinderance; this would be in turbo applications above 12psi, and in an all-motor build. In a NA build, it would probably be the last thing you would do to get more power, since this long tube design with the intake manifold butterfly is rather effective at making decent power.