i don't know how effective it can be. the concept is there. i can't tell you it's 80% effective when you are going 30mph and only 20% effective when you are going 75. but what i can tell you is that if your engine does catch fire, it usually shuts down. then you stop your car and say... wtf is going on? then you see the smoke come up from the engine bay. if the thing works, it will fall onto the engine and suffocate the engine from the top. air still will get in through the bottom, but if the flames aren't allowed an upwards path, it will more than likely lose some of its intensity.
removing it or not is ultimately up to you. the chances of your engine catching fire are almost zero, so it won't be that big a deal. personally though, i wouldn't do it.