This info was taken from Pro Weld.
With very few exceptions, almost every used 4-Valve head suffers from extreme carbon build-up under the valve head and up the neck. This build-up robs more power from your engine than any other single problem I can think of. More than poor timing, a weak fuel system, bad O2 sensors or even a out of calibration mass air sensor. This excess build-up is the equivalent of a plugged air filter and valves that only open 2/3's the normal rate !.!.! If you have a 4.6L 4-Valve and the performance just seems to have gone away, then you most likely have extreme carbon build-up. The supercharged engines were the worse, the combination of low miles driven, too many people that like to hear that big blower, and excess oil mist from the valve covers that were not filtered all helped to create the problem. In really bad instances, mainly worn engines, the oil mist (blow-by) can actually clog the small passages in the inter-cooler which sits below the blower and further starve the engine for air,
Long story short a catch can will filter most blow by that can lead to this problem s/c or not.