At the local Nissan dealership, they're selling something called Engine Fog. It cleans the engine and they do a fuel tank treatment. The fog removes deposits. There's also an engine mist that cleans the air intake system (throttle body).

Is this worth doing?
Will it improve performance?

 

Cleans the inside or outside of the engine?

Cleaning the outside of a dirty engine can help it run cooler, getting back lost HP.

This may help you get back lost HP if you have lots of crap in your engine. If you use the right octane, name brand gas, I don't think this will do anything worth noticing.