Hi Mat,
I'm afraid Mat that what your friend told you is not exact.
Bringing a bike into France from a EU country is free. You will however need to clear customs. That is free but can take from 24 hours to several weeks depending on the local customs office.
There is no limitation for use with a "carte grise de collection". It is in fact, for the moment quite the contrary. All larger French towns are banning older vehicles, unless they have a carte grise de collection in which case, you can enter the town.
According to wether you choose to register the machine normaly or as a historic vehicle, the amount of paperwork is the same, only not the same papers or the same people handling them.
If you choose a historic registration, you need to go through the FFVE (Fédération française de véhicules historiques). They will need the customs declaration, the documents from the bike's country of origin, a bill of sale and a photographic dossier of the machine and its numbers.
If you choose to register normally, you'll need a document from the importer stating that the machine corresponds to the type homologated in France which is a complication if a more recent machine develops over 100 HP when that wasn't allowed in France. They might also ask that you have a headlight dipping to the right and a speedo in km but they might not because as you know there is no technical inspection for bikes in France, or road tax, not like some other places.
Paul