That's the stooopidest thing I've ever heard.
The plenum spacer allows the engine to breath better, the same as an enclosed intake, HFC or an exhaust system. A free flowing exhaust allows for more fresh air into the cylinders too, ie intake and exhaust both complement each other so to say the above statement is ridiculous.
The ecu does not dial back gains shown from bolt on mods, all the ecu is trying to do is maintain the factory mapped AFR which is fine. For example high rpm 13.5 AFR with standard intake and exhaust denotes standard power all things being equal. The same AFR with bolt ons creates more power.
tuner maps a standard 350z and logs the figures. Uploads the same map to a full bolt on zed, which would make more power?
David