No, current cache needs to be btrfs like it says on the notes.
Yes, unless something goes wrong, that's way it's always good to backup any important data.
Yes.
Yes.
It's SAS2 but unlike most SAS2 expanders it's SATA2 max, not SATA3 (and it requires a certain firmware, IIRC 1.80+), so half the total bandwidth of a SATA3 expander, 1100MB/s for single link, twice that for dual link.
I don't know of any 16i based on the SAS2308 chipset, there is a 16e, the 9206-16e, there are SAS3008 based 16i HBAs, but these are SAS3 so more expensive.
I'm seeing the same using the SASLP, looks like some issue with diskspeed and this specific controller, real world bandwidth of the SASLP is around 640MB/s, so just divide that by the number of disks you have connected and it will be max speed during check/rebuild.
The only plausible explanation I can think of would be that the parity disk got disable on shutdown during array stop and there was no time for notifications to go out, because the array won't autostart with any missing disk (including parity) and you definitely get a notification if it gets disabled during normal use, in fact you usually get 2 notifications, one about array errors and another about the disabled disk.