In that case, we are left wondering why you would even be looking at an 8-bore action if its for a roe/fox-gun build?
Have you had a double eight in your hands? They are big....

Having said that, I have bagged a few feral cats in the Australian bush with nitro doubles over the years, up to and including .577. They were no less dead for having been hit with bigger-than-needed projectiles!

If its just for fun-shooting, I'd still be looking at the 8-bore option. You gotta admit, that would be a pretty classy gun to uncase in front of your friends at the range!