Hi, I want to add a couple more comments since I have this W680 board.
I think it's a good decision that you plan to put the LSI SAS 9300-8i on PCIEx16(G5)_2 because this x16 slot pulls 8 pcie lanes from the cpu. As one member mentioned, the W680 chipset limiting factor is the DMI, which is at pcie4 x8 bandwidth, but whether it's gonna be a bottleneck, it will depends on your use case and typical workload.
Here is the implication: the mobo has four sata ports and one slimSAS port. The slimSAS port can be put into SATA mode and support four more SATA drives. In total, the chipset allows eight SATA drives. On top of it, M.2_2 and M.2_3 pull pcie4 x4 lanes EACH along with other components like built-in NICs, USB ports, etc. - all of them need lanes/bandwidth on the W680 chipset. The LSI SAS 9300-8i on PCIEx16(G5)_2 will bypass the chipset and talks directly with the CPU. In my case, I like mirrors. I have one hdd/sdd in the mirror connected to a SATA port via the chipset and have the second hdd/sdd in the same mirror connected to a LSI 9207-8i on PCIEx16(G5)_2. This way, I feel the load will be better distributed and probably doesn't saturate the DMI. Something for you to consider.
I also looked into the Asus M2 hyper card, but unfortunately, you can only get 2 out of 4 slots to work on this motherboard because it cannot do 4x/4x/4x/4x bifurcation. As you probably know, asus has a webpage that documents how many slots it will work on this mobo. Also, in my opinion, the design of this hyper card is bad - it's way too long. There is no need to be this long.
For PCIEx16(G3)_1 and PCIEx16(G3)_2 where you have a question mark, perhaps you can consider a 10Gbps NIC card down the road, which needs x4 lanes.
If I remember correctly, W680 has a total of 28 lanes. Please refer to the chipset block diagram attached.