I can't think of any aural benefit for a horizontal in-wall speaker. The horizontal off-axis response will suffer considerably, also adversely affecting early reflections at the primary listening position. You should either:
* Use a vertical center in-wall. Just leave the lights off till you get used to it, but enjoy better sound in the meantime than you would have otherwise.
* If you absolutely must have a horizontal center, then ditch the in-wall center and go with an enclosed 3-way. You might not want to go with the HTM61 specifically, though. Looks like there's about a 5dB dip right smack dab in an important range for voices.
See measurements.