I definitely agree with those that think that the SB needs a range increase.
In addition to a range increase I have possible solution to fleets just going around the SB and on to the next system. A SB could greatly slow ships speed that are trying to bypass it. There are a couple ways you could implement this idea (keep in mind I am completely ignorant of the coding required in any of these solutions.)
1. As large as they are it is reasonable to think that they would create a significant amount of their own gravity. If I'm not mistaken I think that I saw somewhere that ships travel faster to the center of a grav well and slower when exiting it. (I’ve never really noticed it) A SB could act the same way having it's own mini grav well within the grav well causing any ship traveling away from it to travel much slower. If it would be easier you could make this an ability I would think for realism sake you would want it to affect both enemy and friendly ships.
2. The second and maybe better idea would be to make a "tractor beam" type ability that would slow or stop only enemy ships or even pull them in toward the SB for a period of time preventing them from getting past it easily.
Ideally you would want whatever effect you use would need be the same or larger than the attack range.
Like I said I'm not sure the details or balance issues involved, but since there are already slow ship speed abilities in the game, I would think that that adding this as an ability wouldn't be to hard.