Huge gap? The projected difference between the 3090 and 3080 performance is only 20%. That is smaller than the gap between the 2080Ti and 2080.
With my shunted RTX Titan, I've got the OEM board running 2160 MHz at ~450 watts which is higher than most 2080Ti water overclocks. IMO, if you shunt a FE/reference board, the difference between it and something like a FTW3/Kingpin could virtually disappear. Largely due to NVIDIA using quality components on the FE. Just use a really good power supply to feed the 2x 8-pin and you won't have a problem with power.
Gap in cost...