Problem with in-walls is moving them.
On-walls or freestanding gives you flexibility for placement, albeit not as clean an install look.

Price wasn't listed in the specs, so i would likely go with what John is suggesting unless moving to a M22 would be within budget. I'm still more of the opinion to use a horizontal centre unless your setup allows for an even placement of the drivers toward ear height. This matters less with a smaller speaker config, but one could still buy a much larger, wider horizontal centre with two smaller mains.
_________________________
"Those who preach the myths of audio are ignorant of truth."