First credentials then I climb on my soapbox. I was a cop for 10 years, spent more hours gaining certs in SD, advanced whatever, long gun training, expert ratings yada yada yada than you can believe. I lost 1 'quick draw' competition in my career to Ara Maljian, feel free to google him and see his current national ranking in competitive shooting as I don't keep up
I spent much of my adult life clearing buildings, houses, and being alone in the dark with high stress situations than I care to think about. I spent my entire life from 12yoa in training and competitive fighting coupled with extremely fast reaction time and pistol creds which allowed me to leave my pistol in my holster 20x longer than your average cop. AND I wouldn't do that job in a real city.
Now I do real estate
The more garbage you strap to a SD weapon, the less useful it becomes. If you use a flashlight, leave it on long enough to find the lightswitch (it's your house though you should know where it is without one) to level the playing field. If you search with a flashlight you become a target to a hidden enemy. If you flash it on and off, you're still a target but one with screwed up night vision. If I'm the badguy in that situation I will own you in more ways than you can imagine.
A laser is fun and it works both ways. Unless you have a target all a laser does is point back to you making you the target. I will own...
If you plan on using a long gun you have 2 options. Short for clearing, or longer if you plan to sit in a room or at the end of a hallway and set ambush. If you plan to hunt, you want a short AR. Superior magazine capacity and faster follow up shots. If you plan to hunt you MUST know how to handle a fight with someone who has a superior leverage advantage on your rifle. Normal rifle is great for shooting, piss poor if someone grabs it as you clear a corner. And the muzzle will give a handy lever waaaaaay before you clear the corner. I will own...
If you can't hit it with a rifle at close range, you can't hit it with a shotgun at close range. Inside a typical home the average spread from a minimum length shotgun barrel with an open choke is maybe 1-1.5" If that's all that's separating you between a hit and a miss, the hit is useless. A shotgun is no more user friendly for the inexperienced than an AR and you are better off with 30 massive muzzle flashes than with 8.
Out of all the times I ran the action on a shotgun, never once did it change a badguys attitude much less end a fight. Great in movies though.
More than likely a badguy is going to have no training and go off of brute force and intimidation. But there's the chance that he's as skilled as I am. Prepare for the worst, hope for the best.
Having said all that, the AR I used to clear countless buildings sits down in my gun room next to my shotguns that get used for hunting little birds. The only thing I count on for self defense is a glock .40 that I can clear corners with, shoot anywhere from the hip to full extension to 25 yards and doesn't even have a night sight because I can tell from body position that I'm on target from combat distances. In the unlikely circ. that it fails, I can clear it and be back in action 20x faster than I can with any long gun I own.
So back to the original question I'd stop at an extended tube and shorten the shotgun as much as legally possible. Loaded with 00, birdshot is useless on anything other than little birds. If you want to stress about over penetration you shouldn't be using a firearm regardless. Stick to the poo-stick. The poo stick ALWAYS works.
http://www.ebaumsworld.com/video/watch/761807/