A larger but slower cache would be a downside for an application that did not need the extra capacity. Now it's cache accesses are slower with no gains anywher

A larger but slower cache would be a downside for an application that did not need the extra capacity. Now it's cache accesses are slower with no gains anywhere else. Of course if you don't have enough memory at a given tier and are overflowing in to the next adding more will always make things better, but once you hit that point adding more just adds cost at best and might actually make things worse if it's slower for the extra capacity.