RTX 3090 FE: I had the card for a few weeks: throttle easily in ETH mining, no matter the setting, exepting heavy undervolt/underclock for the core and underclock for the memory: 105Mh/s-108Mh/s @250-260W is doable. Works OK for Conflux, with carefull settings.
RTX 3080 TUF: No problem with ETH mining with core undervolt and memory overclock: core at 1150-1200@0.725mv and mem @ +1200 (10451) gives around 100Mh/s @ 213W. But, it needs appropriate case cooling (when I had both card in my case, the TUF was throttling from time to time).

Overall, RTX 3090 is not appropriate for ETH mining (even without taking into consideration the price): the doubled memory chip on both side are difficult to cool and leads to an increased power usage.
3080 TUF has appropriate cooling on the mem chip and is OK for ETH mining.