Overlay is in non technical term another map. What you have added isn't. 2610's / Nav 2's can't add speed cameras like you have. They can only use an overlay, which gives no extra warning than if you were approaching eg a petrol station. That is it shows them, but doesn't warn you. Problems can occur with the sheer numbers of them making the roads disappear at certain zoom levels. This doesn't happen with the POI's in my experience.
HTH