Personally, I would never scope anything over .375. Good express sights are all you need.

If you decide to do it, you'll find that many will recommend a pivot mount for it that is cheap and readily available. Don't bite, and don't try to save money. Send it to J. J. for claw mounts. They're vastly superior and worth every thin dime. On a large bore double, mounts need to be strong and detachable in less than a couple seconds. Claws detach faster and are stronger than any other detachable mount.

If that's what you're being told, you need to find a new "gunsmith", because the guy you're using isn't one. The sights on a new Chapuis are normal height. If the gun fits you, the sights should align perfectly with a fast, natural mount. If this gun doesn't do that for you (if you're too high on the sights when the gun is mountly quickly), then IT DOESN'T FIT - because it doesn't have enough drop for you and needs to be bent. It has nothing to do with the sights being too "low". A flip up leaf should NEVER be used for a standing leaf. The suggestion that taller folding sights need to be fitted instead of correcting the poor stock fit is a hoot. This is one of the reasons that I suggested to you that buying a new gun only made sense if you ordered it built to your dimensions rather than buying it off the rack.

The double rifle, especially so the large bore, is an iron-sight gun that is occasionally scoped. It should be fit to the shooter for fast, natural alignment with the sights. That fit won't be ideal for a scope, nor should it be, as the scope on a heavy double is an auxiliary sighting system, not the primary.

Don't let anybody screw with the sights just because the stock doesn't fit you!