In Windows 8.1 Storage Spaces, there seems to be pretty solid evidence that 2-way mirrored storage spaces have performance that improves significantly as more disks are added to the pool as long as the resulting virtual disk is configured such that the number of columns is equal to half the number of physical disks. However, these multi-column volumes come with restrictions on flexibility, since you are then forced to add/remove disks in sets equal to the number of columns. So if I'm building a home server with 3 to 5 disks of different sizes, a single-column volume seems to be a whole lot easier to manage.
However, I haven't been able to find any benchmarks anywhere showing how performance scales with the number of disks in a single-column 2-way mirrored storage space volume. My guess is that there should still be some performance increase, particularly for reading large files, if storage spaces disperses each files's mirrored columns evenly amongst different drive pairs since reading the whole file would then still hit all the disks in the pool, but I'm not sure if that's right. Does anybody know for sure?