I would disagree completely. If the point of the cache was simply to receive large files I might as well not have a cache or just use HDDs as cache. The entire point of having a cache and using SSDs, at least for me, is to avoid the slow write speed that I would otherwise encounter with HDDs when writing many smaller files.
While I don't have any data I also feel like the write speeds now, after having added 2 more SSDs to the cache, is basically identical to what it was previously when only 2 SSDs were used in RAID 1, so that adds further to my belief that there is either a configuration error or a bug.