What is a good way of layering zfs filsystems to manage unpredictable future workload that doesn't severely compromise performance?

I've been playing around with ZFS and using sparse files as virtual devices to learn how to use the zfs tools. It seems that there isn't much of a hit in performance by creating a raid pool out of sparse files which are on top of JBOD filesystems that are located on different devices. So I'm wondering if this is a good way to have a flexible system going forward seen as it's not possible to shrink zfs filesystems. So for example I have four disks. Each one has a single device filesystem on it. I create 4 sparse files, one on each filesystem therefore disk. The load is spread across the devices and they are also providing redundancy. Until the raid array fills, I still have all the rest of the disk space for some things which may not need raid such as temporary bulky files. I expect that there would be a major performance penalty when both types of filesystem would be busy at the same time. Are there some more pros and cons of such an approach?