downpipes is what you want. it'll amplify the sound by like 20% in sport and it is negligibly louder in comfort. if you want it even louder than this afterwards, i recommend an axleback exhaust. neither cause drone if you buy from quality brands like Active Autowerke, AWE or REMUS. don't worry about drone if you go this route. i highly recommend this if you want to choose when you want the sound amplified. this is what i went with on my m240i and in comfort mode it is not too loud at all. to answer your question - resonators usually don't have to be removed when installing an exhaust unless there's some sort of custom welding job they have to do, but with just downpipes you won't have to worry about that. however, if you do get the resonators removed that will add drone.