Andy, I don't have an answer for you but a few words. It is not the size of the speaker. Don't be mistaken by the amount of inches. The whole inspiration of the LABsub was one that was very specific, to produce down to a certain frequency in a certain size box. From there the box was made and the minds behind the project were fortunate enough to be able to spec the speaker that worked the best for that box when Eminence stepped in to support the project. I think what I am trying to say is that the LABsub is the most "perfect" DIY sub out there, there is no other DIY project that has a driver made specifically for it, at least not to my knowledge. Until the worlds best horn designers provide us with a plan as we are lucky to have with the LAB 12, it just isn't worth it to even try to find something better in the DIY horn range. Now does that fit you need? That is up to you. Used LA400 are most likely cheaper then building labs.
The main point is don't get caught up on the tool to define the outcome. Define the outcome first, then use the tools to make that a reality. My 12v drill is much better then an 18v one, you know why? Because I get the same outcome, it's lighter, and I like it better. The size of the battery has no effect on the tools job to put in a screw, it's how I use the tool to get what I want. Sometimes though that tool isn't enough, so you know what I go get a cord and a real drill and use a different tool.
Maybe Eminence isn't the answer at all. I think on this board someone used to have an animation of a square peg trying to be put in a square hole. Some to think about too is this, what is more efficient a bunch of smaller things or one big thing. There is a reason why most sub cabs are the way they are. You could put 12 18's in a cab and it would work great. But why don't they do that. You might only need 6 of those subs for the job, then what do you do take the whole thing. If the concern is that a 12" is too small and a 15 is better think of it this way, take a cab the basic size of the LABsub and see if you can put a 15 in it, doubt you will ever get 2 in there, then look at a LABsub and compare the cone area of that single 15" to the 2 12" that fit, which is going to be more? I think this is going to be true for just about any horn design. If you are really worried about the difference still try this, listen to 4 or more of them properly powered and I think the Lab15 will never be thought of again.