Okey doke. Well, I got mentioned by name, so how can I refuse to weigh in?
First, I've not heard these ATC speakers. Can't weigh in on those. But at $1000/pair?! Seems like they're a steal.
Anywho, speaking on things I DO know.
Are you looking for a 5 speaker music system? Or is this primarily a 5 speaker movie system?
Either way, if you can have your centre speakers at ear height and have all 3 front speakers oriented the same way, yes, that is ideal. It usually requires an acoustically transparent front projection screen, or sacrificing viewing quality by having the TV way up high. So that's why it isn't common, and horizontal Centers are all the rage. But if you can make it work with audio as your #1 priority, 3 identical speakers all oriented the same and at ear height is indeed the best.
The surrounds are a different story. If this is a 5 speaker music system, then yes, identical speakers to the fronts is ideal. For music, they should also be at eat height, and 30 degrees to the rear. In other words, if you drew a straight line out to either side, 30 degrees towards the rear of the room from those lines is where the surround speakers should go for music. This is the speaker layout intended for SACD and DVD-Audio.
If it's movies and TV though, it's different. The goal there is to recreate a movie theater's sound, where there are multiple surround speakers on each side wall, all way up high. Surrounds for movies are meant to "blanket" the audience and are mostly for ambience and the occasional sound effect. They are supposed to be diffuse.
You keep talking about imaging. Surround speakers do not image. Even for music, they're either used to place you in the live venue as though you were an audience member, or they're used to place you "on stage" in the middle of the musicians. They don't image like the front speakers do. Mainly because we humans don't hear too so good when sounds are coming from behind us!
Anywho, the choice is pretty easy. 5 channel music? Go 5 identical speakers all at ear level. 5 channel movies? Go with diffuse surrounds mounted up high and directly to either side. If you want to use one system for both, that's where THX Music Mode comes in. It uses 7 speakers to mimic 5. Two diffuse surrounds directly to either side work in conjunction with two monopole surround back speakers behind you to create the illusion that sound is coming from the rear corners instead, where the surround speakers would be placed in a 5 channel music setup. With careful placement and aiming of the surround and surround back speakers, it actually works pretty well! Again, because we don't hear sounds behind us all that well to begin with!
For the speakers, I'll just throw another option onto the heap:
If you can fit their width and depth, consider using 3 of the Ascend Sierra Tower Horizon Center speakers with the RAAL ribbon tweeter upgrade. Most people don't even think of using 3 horizontal speakers, but the Sierra Horizon images wonderfully, and it's front ported design means you can place it closer to the front wall than most other speakers. Although these sealed ATC bookshelf speakers would do that, too!
Dave over at Ascend has also made several custom versions of the Horizon Center, including a "low profile" version with the tweeter and mid-woofer side-by-side. This design could easily be made vertical. Although you'd end up with quite a tall "bookshelf" speaker that might not fit your space constraints.
Regardless, you should talk to Dave. He can customize those Horizon cabinets in many ways. And the RAAL ribbon tweeter is glorious. If you liked the Focal Beryllium tweeter ( and I know you did), the RAAL ribbon in the Ascends is how you get that level of quality, and actually even better transient response, if you can believe it! All at a lower price. About $2700-$2900 for a pair, depending on the finish.
Check Ascend's forum for all the info on the Sierra Tower Center/Horizon. Dave even has a YouTube video of two horizontal Horizons being used as main Left/Right speakers
