Ken my speakers are 8ft apart..no problems. I've listened to 5 inch single drivers that were 10ft apart..no gaps.LOL
You have nothing to worry about in that area.You would be surprised with how wide the stage can be with some 4 inch drivers..probably amazed .If you haven't experienced it before , your in for a shocker!!
Infact you'll spend several days trying to figure out how the designer got so much sound out of one driver. I find it quite amazing ...that's why I'm constantly looking for and reading about the designs of these speakers.
I figured the FTAs would be too big. It was more for field trip experience.So you can get an idea of how some of these different implementations using a wide range driver sound.
BLH or backloaded horns have their strengths. They play a little louder before compression than the Transmission line designs.The BLH is also more efficient. The TL's are not as efficient but have much better low end response.In order to get the same low end response the BLH cabinets must be huge.
In your and my position the TL is where it's at. The cabinets aren't overly large but don't sacrifice low end grunt.As you look through the different types. Look up the prices on some of these drivers.You will quickly realize that a good portion of the cost of some of these speakers. Is attributed to the very high grade expensive drivers some of them use. We're talking $3000 just for a pair of drivers!!
Here's a good site to give you more to think about
Full-range Driver & Loudspeaker Theory.