You need to think more about this. You might have the wrong preconceptions about what a special vdev requires and involves.
If you’re using ZFS replication, it’s unlikely to make any significant difference in replication times. (You might be thinking of file-based tools, such as rsync.)
Small files that are smaller that the dataset’s recordsize property will be saved as special single-blocks that can fit the entire file’s data within. The block’s size will round up to the next “power of 2”.
If you have a dataset that is set to a recordsize of 16M, a 115-KiB file will be saved as a single 128-KiB block, not a 16-MiB block. (Compression will make this 128-KiB block physically take up only 116-KiB on the disk anyways.)
Any files bigger than the recordsize will be saved as multiple blocks, with each block being the size of the recordsize. Compression will still shrink them, especially the last block that has padding on the end.
Anything from 1M - 4M is an ideal recordsize for saving files that don’t have specific needs, in contrast to databases and VMs. You’ll minimize ZFS overhead, boost large file reads and transfers, and get the best compression efficiency for compressible files.