The problem is that you don't know the Thiele/Small parameters of the drivers, so you can't calculate the correct volume of the back boxes. The box has to enclose the correct volume of air and damping. You can measure the T/S parameters with the right equipment, or Focal may tell you, if you ask them.
Certainly the correct size of back box is the way to go. Personally I think firing drivers right into the wall space is an awful idea. So when I designed and built mine, all drivers were loaded by the optimal sealed box and I designed with drivers with optimal T/S parameters for sealed loading. I personally think mounting in wall speakers to sheet rock is a terrible idea. I have to say I think most in wall systems are half baked, and not well thought out. So the old adage applies, if you want it done right do it yourself.
That is my wife's system, designed and built for this large space at her request, and powered with 1000 watts all channels driven. She loves it. I get to play and watch what I want in our AV room. We were both in the AV room this evening.
In wall systems are a little tricky and present there own unique challenges.