As far as the ECU adapting to more air in - more air out therefore up fuellng...the problem is that when its running with wide open throttle it doesnt measure air pressure or flow it just uses a pre-fixed fuel map...this is why people who have filters/exhausts got to Nick Hill/Superchips for a revised fuelling map.
When i was talking to a renault dealer they said, when asked about filters and exhausts, that anything that affects the engine would void the warranty, including the exhaust as it puts more strain on the engine.....we all know its a load of crap, but for renault its a get out clause.
Ta
J