I dont know what performance gains he was looking for, but engine flush is to clean the insides of the engine.
Since you have a lot of miles on, an engine flush would not harm. I suggest you flush the engine, and if you notice any leaks or strange sounds from the pistons etc, add Ceratec for an additional layer of protection inside the engine. Just keep an eye for any leaks.
Sent from my HTC One using Tapatalk