Peak efficiency may be around 50% load, but you're forgetting that this is an 80 Gold unit. While it's at least 90% efficient at 50% load ( the graphs looks to be a bit above that ), it's still ( at least ) 87% efficient at 90% load. You're only losing about 5% efficiency between the two here. That only means it draws a few more watts from the wall. Delivery inside the case remains unchanged.
GPU mfrs' recommended PSU capacities are always inflated to compensate for the mediocre ( or worse ) PSUs that flood the mainstream and OEM space. A 980 Ti will pull at most 250W - 275W, depending on OC, during a heavy gaming load. Torture tests can go above 300W, but no one mines with an ITX box. Adding the 130W you'll see from the rest of a typical i7 system, you'd rarely see this go above 400W. That's an 88% load on a 450W PSU. Tight, but certainly not risky or dangerous. As I constantly remind people, my i7 + 290X test bed runs just fine on a 500W PSU, and that GPU is a lot hungrier than the 980 Ti.
The only legitimate use for this PSU, with it's 600W but very limited cables, that I can think of is a heavily OC'd ITX gaming box built in a case that requires an SFX PSU. I can't find a single Z170 or X99 ITX board that has more than an 8-pin CPU power connector, so the PSU's limitation there shouldn't matter. Split the load 300W for the GPU, 250W for the CPU and rest of the system and you duck in just under the power limit. But again, that only makes sense if you MUST use a SFX PSU. Plenty of small cases support full size ATX units, where you have a lot more selection.
This feels as though either Corsair is trying to fleece some customers into spending more than necessary or they got lazy in expanding a product line by increasing capacity without also taking the trouble to re-tool the cabling.