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I agree with you there and I would expect the 4080ti to be around the same if not better performance than the 7900xtxThe way the pricing currently is, the 4080 Ti will fit right between the 4090 and 4080 and will be closer to the 4090 than the 4080. It makes the 4080 pointless. Buying a 4080 is not a good move as it's so apparent they will be launching a far superior Ti for £100 more.
Or at least be competitive, the problem is they have backed themselves into a corner on how they price the 4080ti so it's possible it will end up being the direct competitor to the 7900xtx but at a higher price but like you say would make the 4080 obselete.
Will be interesting to see how it ends up performing and fits into the stack.