The best solution would be to scratch it all and start over. However, that's such an outlandish solution for this circumstance, so it can be thrown out the window. And, you will never fully get 100% of the spotted bass out of lake without drawing it down and hand picking them out. That will cost valuable man-time and tax dollars.
While I like the idea of a slot limit. However, tournament anglers will claim that they can't be as "competitive" with the limit in place. Say you catch 4 spotted bass, all over 15". In a normal tournament, they would be legal. However, with the slot limit you talk about, it would be a decision on which to keep, especially if they are close in weight. Also, they will complain if they have to throw back a spot that weighs 4-5 lbs., in favor of the regulation, and has to take a spot that is 12" long and only weighs one pound. Personally, I believe that recreation anglers are the strongest component of any fishery movement, and tournament anglers are including that category. With your proposed slot limit, you would be dividing the group into two seperate groups.
I think that a slot limit is the answer, but you have to be careful about what boundaries you set. If you go for the 12'-15" as you proposed, you would have to put it as 5 of either, or any combination there of. Or, set it at one length size............say 13" and let it sit there. Personally, spaaats are spaaats to me, and if they are 12" they are fair game. Tournament or table fare. You decide.
I also think that the DNR and fisheries biologists input should be heavily weighed into any decision. My faith in these people has been shaken of lately, due to the ignorance that some have displayed in the state of Georgia. However, I do believe that they know, for the majority, what they speak. If they say that 12" is the limit, then that tells me that there is an over abundance of spaats and that the harvest limit needs to be increased. (Please read the post that MadBomber posted about this very decision). However, if they agree that a slot limit can be implimented and it benefits table fare anglers and tournament anglers, then it should be done.
You are looking at a double-edged sword. On one hand, you are looking at possibly upsetting the tournament anglers, which are a huge ally in decisions such as this, but on the other you are looking at basically trying to wipe out an entire species of fish for the benefit of having bigger size. It would be nice if you could have both numbers and size, but it seems with spaaats, that happy medium is difficult to find. Carters Lake and Lake Lanier are the only two lakes that I know which have that happy medium, and even they are pushing the boundaries with it.
So, for right now, catch all the legal sized spaaats you can, eat them and share them with friends, and be happy.
Have no worries.