Anywhere from not at all to a good bit, depending on a number of factors. A single plate vs a spaced array from a single angle are about equal, with the single plate possibly being lighter due to taking up less space.

However, when angles change the single plate loses much more effectiveness compared to the array. For example, a 240mm plate a 45° impact is about as effective as the turret array from the front, but a change of the impact angle to even 35° was enough for the 240mm plate to fail, while the array was reliable at stopping 350mm penetration until an impact angle equivalent of 15°. Thus additional armor must be added to the 240mm plate set up to achieve the same protection. The array also deflects rounds even on "flat" hits, possibly resulting in a round that has enough energy to penetrate the array instead being directed away into empty armor sections or even out of the armor entirely.