I schedule scrubs during idle time (10pm Friday - 6am Monday), so the metadata hit doesn't affect usage, but YMMV if you don't have an idle time. I also don't generally let them get that full.
If I was building the array new, I'd probably use a special VDEV for metadata, but they didn't exist when I built my last one of these.
Disk failures don't neatly wait for idle times so investing in a special vdev (which can be added to an existing pool as the article points out) should be well worth it.
If I was building the array new, I'd probably use a special VDEV for metadata, but they didn't exist when I built my last one of these.