So, let's say all stripes are on the same Disk-Group (but on different Capacity-tier devices as this is what SW=2/3/4 does from a placement perspective), if the Cache-tier write-buffer is filling to the point that it is actively destaging data to the Capacity-tier devices then destaging to 2 or more Capacity-tier devices is always going to be faster than a single Capacity-tier device.
A simple analogy: I have wide pipe that can drain 10L of water a minute, but it is flowing into a thinner pipe that can only take 5L of water a minute - this is going to be a bottleneck - but if I have 2x5L pipes accepting the flow then nothing is left waiting.