Yup,
They need cleaned at least every 10K miles.
There have been multiple owners who have dyno'd before and after the cleanings, Huge difference. Car's in good health will begin to scream between 6000-8300rpm's. When they get all carbon'd up it will pull, but not with increased "zest" in that range.
If the car doesn't appear to suddenly go apeshit up top, then it's probably way overdue for a cleaning.