I bought a used Benelli with both barrels in 2004. I shot a wide varity of slugs through it and it shoots Lightfields and Hornady SST's fairly well but not like either of my current guns. I'm not sure what a new Benelli rifled barrel sells for, I paid $800 for the whole deal. My son used it the last couple years but he didn't swap barrels and put the scope back on this season instead opting for Dad's Browning Abolt which is a tack driver but no longer in production(made only from 1996-98). I also have a customized Remington 20ga that shoots almost as good as the Browning but both guns are more expensive than buying just a barrel. My suggestion is(if you can afford it) purchasing a used Remington 870 20ga and placing a Hasting rifled barrel on it. Have the reciever drilled and tapped for a scope base. This would give you a dedicated deer gun and would wouldn't have to switch barrels and re-mount your scope thus having to sight back in with expensive sabot slugs. Down the line you can have the barrel pinned for added accuracy and the trigger can be tuned by a gunsmith. With the advancements in sabot slugs there is no reason not to consider a 20ga which will be much easier on your shoulder plus a good starter gun should you introduce a youngster to the sport. Hope this helps.
PS- I don't like cantaliever mounts, they move around just enough to destroy accuracy in some guns.