I have 4 Nvidia GTX 1080 Ti cards in my rig currently.
I have space for 2 more cards.
Should I hold out for finding actual Nvidia GTX 1080 Ti cards or do the other brands version run comparably well?
what do you mean by "other brands"? nvidia is only the producer of the chip.
For computational power (adjusting for any overclocking done by other manufacturers), performance should be identical.
For quality of other components, and cooling design, brands will vary - except for the "Founders Edition" models, which are all manufactured to precise NVIDIA specifications, and should be virtually identical regardless of brand. The FE cards are also reportedly a bit overbuilt and quite hardy. And if you are packing multiple cards close to each other, the cooling design is superior.
Just paying it forward from all of the really smart people here.