I just watched Legitstreetcars video on cleaning the carbon on a Mercedes v-12 with over a 100k miles. It was ugly to say the least, not only did the valves have loads of carbon, but the intake runners were nasty with baked on oil from the PVC and EGR systems. From working on the 2.5 24 valve Duratec Contour and the 4.6 Mod motor I can say from personnel experience that a catch can makes a difference.