Turns out that contrary to what I understood at first the OP didn't have this HBA+expander combo yet, was just thinking of getting it and assumed the performance, I got one recently and tested it with an LSI 9300-8i and SSDs, note that these results are after updating the expander to latest firmware, it performed much worse without it, similar to what would be expected without Databolt or the PMC equivalent.
Unraid read test (no parity):
RES3TV360 single link with LSI 9308-8i
8 x 490MB/s
12 x 330MB/s
16 x 245MB/s
20 x 170MB/s
24 x 130MB/s
28 x 105MB/s
RES3TV360 dual link with LSI 9308-8i
12 x 505MB/s
16 x 380MB/s
20 x 300MB/s
24 x 230MB/s
28 x 195MB/s
So up to roughly 4GB/s with single link and 6GB/s with dual link, not that different from the LSI SAS3 expander, unfortunately since my LSI expander is on a backplane I can only test that one with up to 12 devices, this combo with SAS3 devices should be capable of around 4.4GB/s with single link and 7GB/s with dual link.
Curiously total bandwidth decreases with more devices, possibly due to the hardware used or Unraid but don't think so, using @UhClem's script only got around 4500MB/s max with dual link (with or without the B option) using 12 or more devices, so there shouldn't be a CPU/Unraid bottleneck, I suspect the technology used by PMC to emulate a 12G link with 6G devices loses some efficiency as you add more devices.