In lieu of sleeving perfectly good barrels for ruffed grouse / churchill method / heavy cover hunting, why not have another set of barrels fabricated?

A gun with two sets of unsleeved barrels is surely more versatile and valuable than a gun that was sleeved solely to give you greater length.

Further, the sleeve job is going to be costly because you're going to need a new rib since the current one has been cut short. All the more reason to pursue another set of barrels instead.