Why larger guns would not solve Surcouf's firing limits

Would fitting Surcouf with 9.4-inch guns make the ship worse?

No, but larger guns would not remove the hull-stability and sighting limits that already constrained useful range.

A 9.4-inch battery would not materially worsen Surcouf's already awkward gunship concept. Useful range depended less on nominal gun reach than on the submarine's stability and ability to see the target. A long-range weapon cannot exploit its ballistics if the platform cannot remain steady or sight a target far enough away. Caliber alone therefore does not determine whether the cruiser submarine can use its guns effectively.