If i only have a rather small bedroom (15sqm), would it be pointless to upgrade my center speaker say from a polk audio RTi CSi A4 to an RTi A6? or paradigm CC-590 to CC-690
Say that I use my system for 90% movies 10% music
For one listener dead center in sweet spot, I don't think a separate center speaker will help to separate center channel content better than a phantom center would, all other things being equal. Meaning, the center speaker must be of the same caliber and positioned as well as the other two front speakers.
But, for people far off-axis, sitting to the sides of the theater, the phantom center doesn't work as well. The two speakers being of unequal distances will make the timing be off between the front and right speakers trying to generate a phantom image. And the soundstage will be more heavily dominated by the near front speaker, with the center channel content being more diminished.
Still, that pain might be less than the harm done by using a weak center speaker with respect to the other front speakers. If you like to run the system louder with peak levels over a 100dB, so you bought two front speakers with a 10" woofer and 6" mid and two tweeters, or a horn, the you will be disappointed by a center with a couple three inch mid-woofers, or even a couple 6" woofers. The center will have become the weak link in the chain.
And it's not just voices coming out of the center. Explosions, orchestral pieces, laser beams, the pounding of horse hooves, etc.
If you use a disadvantaged center speaker versus capable more stereo fronts, you're saying, "I want high output for the front-left and front-right sound channels, but for 60%-80% of the movie (coming from the center channel), I want it to clip, sound harsh and strained and not as loud."
Or, you just don't turn movies up loud, and thus you don't make much use of those capable stereo fronts in cinema mode, and you reserve them for stereo music. Which is cool...but people should be aware of their system's goals and limitations.