First of all, it is a lot better to focus on the bird rather than the sight. That said, you can install one of several models of fiber optic sights made by Truglo (Truglo wing and clay shooting) or Hi Viz. These are very easy to install (on the magnetic models all you have to do is pop them in on top of the barrel or the rib), and they do increase the visibility.
For skeet use the "Skeet" choke, AKA cylinder choke (five stars, five notches). If you don't have a skeet tube use an improved cylinder choke (four stars, four notches). For trap use either Full choke (one star, one notch), improved modified (two stars, two notches), or modified (three stars, three notches). For Sporting Clays, read this article: Single Choke Tube for Sporting Clays
Yes. You can buy Benelli or 3rd party. Check out the Benelli ones on this link:Benelli extended tubes