An identical center channel is the only way to get an exact timbre match across the front sound stage using free standing speakers, although you can get very close with "center" speakers having a coaxial or vertical MT array, driver compliment matching the main speakers (in-wall speakers in that configuration will match because their baffle configuration does).
Your brain forms its impression of timbre from the spectra of a direct sound and what it believes to be the sound's reflections.
Although one can easily make different speakers identical on-axis, lobing from horizontal driver pairs and differences in polar response from the center channel baffle being wider horizontally but narrower vertically mean it won't be exact.
Different low frequency extension will negatively impact phantom image placement due to differing phase lead caused by the speakers' high pass function.
Phantom image placement will also suffer if the cross-over points and slopes are not identical because the left/right and center channels all-pass behavior and relative phase will vary at different frequencies.
Your brain forms its impression of timbre from the spectra of a direct sound and what it believes to be the sound's reflections.
Although one can easily make different speakers identical on-axis, lobing from horizontal driver pairs and differences in polar response from the center channel baffle being wider horizontally but narrower vertically mean it won't be exact.
Different low frequency extension will negatively impact phantom image placement due to differing phase lead caused by the speakers' high pass function.
Phantom image placement will also suffer if the cross-over points and slopes are not identical because the left/right and center channels all-pass behavior and relative phase will vary at different frequencies.