Whether you need an extension or not depends on the circumstances in which you're most likely to use the gun.
If you live in a quiet area, the police respond fairly fast, and the most likely threat is going to be one or two burglars or intruders who will likely run at the first sign you're armed, you may not need it.
If you live in an isolated area and may have to hold them off for a while, a rough part of town, the police are slow, and you may be facing a small mob, you may need an extension.
Just remember in accessorizing a shotgun, the reason a shotgun is so deadly really isn't just the power, it's the speed you can get on target and shoot.
Anything at all you add will increase the weight and bulk and will actually slow you down.
In shotguns speed kills.... the bad guys.
You have to do a case by case "Real World" cost-benefit analysis to determine if the loss of speed is worth whatever gain you may get.
Too many people wind up with shotguns that weight half again the weight from adding what are really things for a range toy, not a true defense gun.
Truth is, in most situations the standard police type 18" barrel plain pump gun is the perfect defense gun.
If you live in something other than "most situations" you may want to add accessories, but remember that simple is better.