If you are willing to spend up to $1500 for the gun and having a shotgun to shoot fowl or clays is important to you, then I humbly suggest that you do it right and get two guns that do their own thing WELL. $1500 is a good amount of money to be willing to spend on a shotgun. The Remington 870 Express is definitely a good gun, especially if you're on a tighter budget. But the fit and finish is nowhere near an 870 Wingmaster or 870 Police. The "parkerized" finish...or whatever it is...on the Express RUSTS and the action is not as smooth.

So, if you really like the idea of a pump action shotgun for clays and fowl, do it right and get an 870 Wingmaster. Pick up a used 18" barrel and keep it on there for home defense and swap out the barrel when you go shooting. Or maybe you'd prefer a semi-auto for fowl and clays. You could look at the 1187s. Or a variety of other shotguns.

Get a Savage 220 F for the deer gun. A 20 ga slug has a lot less recoil than a 12 ga and it is PLENTY enough for deer. You can pick them up new for around $425. Or you can pick up a Marlin 512 for around $300 used in great shape. Or there's also the H&R Ultra Slug for $250 (new). If you really get into using a bow, you can sell any of these guns with minimal loss, but why would you? You may want to hunt into firearms season and find the pressure has increased a little too much to be able to use a bow effectively.

That's just my $0.02. IMO, if you don't have a choice and can really only afford to get a combo like the 870 or 500, then by all means. That's what I did at first, but I regretted it. It's better to have guns that perform well at their specialty than trying to have a jack of all trades (master of none).

As far as a cantilever mount goes, it's probably going to sit a little higher than a mount tapped directly into the receiver.