Wow, thanks for the quick and very detailed responses guys! I'm really grateful
Very interesting answers - I have actually been running it with just the 8pin and it has been fine so far, but I keep getting a Steam VR crash that I thought might have been related to being underpowered (after doing a load of other troubleshooting for it). I'm definitely not doing any overclocking - pretty much just using the machine for playing Beat Saber and other vr games on my Index in the living room!
Another thing was when I was running Beat Saber, GPU-Z said my PerfCap reason was 'pwr' (Limited by total power limit).
I'm running a Corsair 550w psu, which I think is 50w under the recommended number for a 980ti.
I didn't originally intend on using that card with that psu, so just bought a 650w one but it only had the 1x8pin cpu, so wanted to return it and get one that has enough cpu pins in addition to wattage to power that card. But, because I'm only using a single graphics card in the machine, I don't need one with stupid watts, but feels like if I'm buying one for this machine's dedicated VR purpose, I might as well get one that fully powers the cpu?