Hi Gregor,
the speed of the FST is the sum of all disks. If one disk is slower read rstreams served by this disk are slower. When you write or use erasure coding you couple disks, so you should couple disks with the same speed between nodes.
Corollary 1: that is good practice to have the same disks behind FSTs
Corollary 2: yes it increases the speed for the streams served from this disk when reading replicated files. When reading or writing erasure coded files, the slowest disk defines the baseline.