I did some digging, would have been easier if you had posted your dig, as I do seem to confirm your findings partly. I'll clarify
Below you'll find three pages from SI, which you've obviously seen before. One page shows a BIOS update for RAID 5, this is true. However I'm afraid you're out of luck, the update needs to be embedded in a general BIOS update released by the motherboard manufacturer. In our case Epox, which doesn't exist anymore.
The DOS update utility is for PCI cards using the SI3114 chip as controller.
It is advertised in their product catalog supporting RAID level 0,1 and 1/0. This is probably why I assumed that the 8RDA6+ Pro would not support RAID 5 correctly.
Obviously we shouldn't really wait for an update out of official channels but there have been several mods made by Merlin, search this thread if you're still interested after reading all the rest:
8RDA6+ Pro Romsip Mod
Merlin released one BIOS mod which contained RAID 5 support, which is highly irregular as he normally only mods general system bios parameters. I tested a few versions for him, none really better than the first mod.
The modding situation is described in the 2nd post of the thread mentioned earlier but as I said earlier, I never tried the version with RAID 5 support embedded. I didn't really feel like losing the volumes
I mentioned Andy earlier as he might know more about getting the RAID 5 bios in the SI chip and he also hosted the bios version with raid 5 embedded.
So, long story short... If you really want RAID 5 support and feel like going for a serious long shot I won't keep you
Any more questions, by all means ask
edit: About the command queuing...
I really don't think it's going to be a problem. Command queuing on SATA disks sounds like NCQ or TCQ, which is not supported by the latest official BIOS from Epox.
I also ran 2 different types of disks in RAID on both the nvidia or si raid chip. Only problem I had with it was a busted nvidia chip on the board, doa.
SI 3114 product page SI product listing
The one we're interested in is at the bottom of the document.
The Firmware/Driver/utilities update page