Another observation I made when installing the Sapphire Radeon RX 580 Pulse: this particular card has heatsink on the back of the card
In a Blackbird, the 8x slot is on the fan side of the GPU, so the rear heatsink on the RX 580 will not clash with another card.
In a Talos II Lite, there are only two slots and the 8x slot is behind the 16x slot, opposite of the Blackbird layout: therefore, if you put something like a HBA with a front heatsink into the 8x slot, it may be too close to the rear heatsink of the GPU
I tried putting the LSI 9207-4i4e behind the Sapphire Radeon RX 580 Pulse and I found they don't touch but the gap is less than 2mm
The only way around that appears to be choosing another RX 580 variation or using one of the other strategies described above, e.g. a PCIe riser or an 8x GPU