Just to confirm, in France, horses are coupled according to ownership. If the Aga Khan has four runners in the Prix de l'Arc de Triomphe, they collectively will be the favourite on the Pari Mutuel. So if you fancy something that the Aga Khan owns for the race, back your named selection with a British bookie so that you get the real odds. Obviously if one of his other contenders wins, you lose whilst bettors on the PMU will still collect.
If you want to follow a particular stable then you have to back their runners individually in this country and that seems perfectly fair, so I would not favour any sort of trainer/owner coupling in the betting. Over here you get exactly what it says on the tin.