It does need some airflow around it, remember these controllers are designed for servers with excellent ventilation, I usually have a 120mm or larger side case fan blowing air on them.
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'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.