I'm in a similar situation...
I bought a used 870 for $350 about a year ago. It has the 18.5" barrel, an adjustable (4 position) A4 style pistol grip stock and a 7+1 mag extenstion. I added swivels and a sling for $20.
I took the gun to the local clay range and asked if I could shoot it there. The answer was "No; you need a 22" barrel and cannot have a pistol grip." So, I bought a 28" Vented Rib Remington barrel with a screw in modified choke for $199 from the pro shop. I also bought a stock synthetic 870 stock from ebay for $30. So for about $599 I have a shotgun that can be configured in about 30 different ways. For everything from "You got to be kidding, a pistol grip 12 ga. with a 28" barrel" to a small tatictal SD/HD shotgun, to a long 28" barreled clays gun.
On Thursday I will take my first sporting clays lesson. The instructor said he will teach me on the 870 and also let me try his semi. He is excited since he shoots lefthanded and so do I. I will let you know how it comes out!
I have only recently shot shotguns at a Cowboy Action Shooting event where your choice is pump, lever or SxS. These people are FAST! They pick up an empty shotgun, load two shells, shoot two targets, and then reload and shoot two more targets. One event I saw on TV had the second shotgun target send a flyer when it was hit. In that case, after you hit your second target, you had to RELOAD two shells, acquire the target, break the clay and then swing down to the fourth static target. All on the clock, shortest time wins! If you want to see some seriously fast pump and SxS shooting, watch Cowboy Action Shooting. Lever action shotguns are interesting but simply too slow compared to the pump and SxS.
So, since I want to do both Clays and Cowboy, I figure I will learn to quickly manipulate the pump on the 870. If I decide to get serious in sporting clays I will probably get a purpose built gun.
So lets see... 1897 12 Ga for Cowboy, 1895 Lever Rifle for Cowboy, a pair of single action revolvers for cowboy, the 870 for home defense, and a autoloader 12ga for sporting clays. I probably will need a 12Ga for trap after I get in to the sporting clays, and another one for skeet. Yes, my wife is going to love me buying all of these!