You're pretty wrong regarding performance here. In ZFS, performance is directly bounded by the number of vdevs given to a pool. If you make one 90 drive vdev, you are more or less limiting yourself to the iops of a single disk. It's a bit more complicated than that, as your throughput increases.

At work where we use ZFS extensively we more or less use # of vdevs * I/O performance of a single drive = total "worst-case" IOPS prior to caching. Caching of course is where ZFS will start to shine - it will win no performance crowns when put up against a "traditional" RAID6 array with the same number of spindles.

This is also why most production implementations of ZFS utilize many sets of mirrors. For example your 90 drive example would be 45 sets of mirrors in our environment, granting roughly 45*120 iops (if NL SAS) total peak capacity. We avoid RAIDZ largely for the reason it would drastically lower the total # of vdevs, and performance falls off a cliff in that situation.

ZFS (not on Linux) seems to handle up to around 300 drives fairly handily before you start to run into issues I've found. 280 is our current max cluster design.