Techspot/Hardware Unboxed recently did a comparison of the GTX1080TI and the RTX2070 Super:
https://www.techspot.com/review/2017-geforce-1080-ti-vs-rtx-2070-super-vs-radeon-5700-xt/
The RTX2070 Super isn't faster over a whole lot of games. It's faster in some newer games,but the GTX1080TI is faster in some very popular games. Techspot/Hardware Unboxed tested 35 games with an overclocked Core i9 9900K.
The RTX2070 Super also only has 8GB of VRAM,while the GTX1080TI has 11GB of VRAM,so if you are into modding games at higher resolutions,8GB can be an issue.
The RTX2070 Super at its current price-point is really overpriced. You are looking at least at £500 and above for a new one now. Even the bog standard RTX2070 which was around £350,is now £400 and above. TPU compared the best RTX2070 Super available recently:
https://www.techpowerup.com/review/galax-geforce-rtx-2070-super-hof-10-year/27.html
This pre-overclocked model was 19% faster against an RTX2070FE,and the RTX2070 Super FE 12% faster at qHD.
Also HUB,said from their own experience,the AMD issues they had seem to have been mostly fixed now:
The RX5700XT can be had for around £350,and the RX5700 for under £300 IIRC.
Personally,if you have an older backup GPU to hand,its probably better to just turn some settings down and wait for the new generation GPUs.
The fact is you will be spending £500 on a GPU which at best is probably 10% to 15% faster in some games over a GTX1080TI,upto a similar amount slower in older games/games based on older engines and those which are DX11/DX9,there is less VRAM,etc.
You might get some RT and DLSS additions,but realistically for £500,its not good enough IMHO. On average you can get £150 to £200 for a broken GTX1080TI on the famous auction site,but you will have to remove fees and postage for that. Any more than that is usually low feedback bidders,which at least to me looks a tad dodgy.
Even if you get the upgrade cost down to £300 to £350,it seems a bit mediocre.