For cooling 1.8KW, 6X 360Radators should do the job, but possibly with 15-20C delta in water temps, If its 10C above room temps now, you can safely say it will be doubble that.
Smaller output peltiers is good for a few resons, heat trasnfer needed would be less, power disipated through a larger surface area.
You get same performance as a single higher power peltier.
If you have 900W of chill now, upgrading to doubble that, which is 1.8W of power usage worth of peltiers, at full volts.
It should be more than capable of getting you subzero
Otherwize, phasechange would possibly be the onlything that could satisity you
Conseqeusnes would be the need for more waterblocks, more cables.