MAXPHYS parameter of kernel controls maximum I/O size supported by the system. The default value of 128KB is heavily restricting tape applications performance and compatibility by restricting maximum sector size. Also for HDDs ZFS tries to aggregate I/Os up to 1MB, which is not as efficient if they will be later chunked into 128KB pieces, which just overflows HDDs queues (only 32 tags for SATA), causing starvation. And in general for modern HDDs 1MB transfer time is comparable to average seek time, that should make quick IOPS vs bandwidth calculations easier.
The downside of it is some additional memory consumption, but hopefully it should not be a problem for typical NAS systems.