That is quite a bit of material, isn't it? There is no way anyone could expect an engine to function as it should with it being gunked up with that much carbon. This car had 69800 miles on it and did not have any aggressive modifications only a rear section exhaust and filter.
As for the effect on performance, the baseline on a Mustang all wheel drive dyno before the cleaning showed 287 horsepower to the wheels and 240 pound-feet of torque to the wheels. After the cleaning these figures change to 329 horsepower and 266 pound-feet of torque. A night and day difference.
There are measures that can be taken to minimize carbon buildup such as oil catch cans and methanol injection but this will be an issue no matter what and one owners of direct injected engines should monitor. A good cleaning can make the car feel like new especially if one gets the impression it has become sluggish over time.
Valves before the cleaning:
Valves after the cleaning: