The question is how many times will you be listening outside of the sweet spot? How many listeners of the system will even care about that imaging?
A phantom center can deliver good imaging at pretty much just one listening position. A poor center speaker will provide imaging outside of the sweet spot, yes, but it will have crappy sound quality otherwise and will deliver subpar sound to all listening positions.
The only way to get good phantom center imaging outside of an equidistant position from the left and right mains is through
time-intensity trading where you take some speakers with narrower dispersion patterns and position them with an hard toe-in to cross in front of the main listening position. Audioholics has a good youtube discussion of it
here. The JBL Studio 590s may have the correct kind of dispersion pattern to pull that off, so it is worth looking into for yourself.