Personally, I think a set of 20g barrels can make a good partner to the rifle barrels, but agree with Sinner that both barrel sets must be set up to balance and handle correctly.
I sometimes hunt on an outback property where the game includes very good sized mountain boars, big billy goats, deer, rabbits and foxes.
I take along a 400-350NE double which also has a set of 20g barrels, and that set up can adequately handle anything on that list.
The gun handles very nicely with either barrel set.
When hunting rabbits and carrying the gun with the shottie barrels fitted, I always keep a few rounds of buckshot and a couple of slugs in my pocket in case I run onto something bigger than bunnies.