... aaaaaand, I've had another drive start throwing uncorrectable sectors - this time, one of the replacements that Seagate sent me! And it's beyond the pitiful replacement warranty period. So I have an HGST NAS-rated drive on order to swap in, and I'll get to see if my resilver performance is any better with the LSI SAS2008 card I've put in. But to confound the results, I also increased the size of the pool to 7 disks from 6, so I can't isolate any single variable if performance does improve. Eh, oh well.
I will say that my scrub speed with the new setup is maxing out at 582 MB/s, so hopefully only good things now.