Many of the v8s are well down on power as the direct injections really coke up, some as much as 100bhp. Many have been cleaned by dealers but will eventually coke again. They need to be driven hard to stop if occurring (possible solution) but it will not cure it if already coked. Best to get it checked by a dealer or someone with a video scope. Poor cleaning can cause engine failure if the deposit is allowed to drop in the cylinders.

Walnut blasting seems to be the way to clean them, terra clean will not do it. Hope this helps