@DigitalAlchemist,
you are forgetting the M.2-slot for the WWAN / a second SSD, which at least takes two additional PCIe lanes (if not three).
I think I have read somewhere that all the current ThinkPad T series has two-lane Thunderbolt 3. The X1 ThinkPads all have four lane TB3, as does the P series (at least P51 and P71, I think the P52s has the same limitations as the T580).
Regarding the screen, I would be careful to take the 280 nits we measured for the T480 as "it will always deliver 280 nits". Lenovo has specced it at 250 nits for a reason and there are several different display-panels used. Plus, screens always vary in brightness. The T480 FHD screen may deliver 280 nits on one unit, it may only deliver 230 nits on another. Its likely the same situation for the T580 FHD screen, which are spec-wise unchanged from the mediocre screens used in the T570.